Rustproofing- Best Practice for Patching

[redblock]

Car in question is an 85 Volvo 760. 

I noticed while doing some transmission work not long ago that any place where the front frame rails have been used as a jacking point, the rubberized undercoating that the previous owner had done is totally flaked off and surface rust is forming. There are two spots, about 6 inches square, that need attending to- the rest of the undercoating is whole and seems to be doing a fairly adequate job.

I won't have time before winter hits to totally strip and coat the chassis in something better, so I'm looking at a couple strategies and wondering which one is less dumb:

1. Mask up the sections that are flaking, then wire wheel/rust convert/POR-15 over the sections that are exposed. I feel like this would just create problems where the undercoating meets the POR-15. 
2. Mask up flaking areas, wire wheel, rust convert, POR-15, THEN use linseed oil to re-wet the undercoating and blend it in to the repaired area. Once that's dry, re-spray with 3M rubberized Undercoating. 

I'm also open to any other suggestions. The underbody will be getting fully stripped in the next year or so and treated to a full POR-15 bath, but I'm concerned about these small sections getting worse over the winter if I leave them exposed. 

[chris y]

Just a few points, there are many ways to do this though.

Wire wheel will tend to shine up the rust, OK for taking the thick off but then you need something abrasive like a flap disc on an angle grinder, or a power file / finger sander. If you don't have these then for the little bit you are doing very coarse emery by hand would work.

Rust converter yes, there will always be traces of rust even if just in pits that it created. Personally i use Bilt Hamber Hydrate 80

POR15 is rubbish ~(in my opinion) there are lots of stories but my own experience is that its rock hard so you cant see rust forming beneath it until it falls off in sheets. Now I admit that this might be less than perfect preparation, but if this product only works in perfect conditions that you can only achieve in a lab then its not practical. I use Bilt Hamber zinc primer but the most popular choice nowadays is epoxy primer, which I have only used once so far, seems good.

don't be putting oil on it before anything that you actually want to stick.

Ideally spray good quality rubberised stonechip, I'm sure the 3M you use is good, I use others, Soudal is said to be very good and good price, I've used it a lot.

Over all of that its up to you, you can go for body colour paint if you want, or any underbody sealer you like.

Just my thoughts.

[steveo3002]

scrape back loose stuff with old wood chisel , wire wheel it and roughen up with sandpaper/grinder

2 coats of epoxy(like jotamastic)...will last a good long time , epoxy isnt fussy like por15

leave as is or underseal it to blend in ...doesnt need underseal to protect though

[redblock]

Appreciate the input- I think I'll steer clear of the POR-15 for now and take a look at a few epoxy products, or if I get lucky and find a US Bilt Hamber distributor I'll consider that as well. Thanks much!

[Deleted]

Ok has many know I run a small independent restoration business so a few things for you consider:

1 - POR 15 or hard setting epoxy mastics don't get a look in at my workshop and never will - they are useless and I can write a book on why - but I don't have the will or the time - you need something that will move - expand & contract with the vehicles structure & that will to a certain extent self heal when impacted - that's why vehicle manufactures use flexible compounds & sealers on the underside of vehicles when in production

2 - Degrease the area thoroughly with panel wipe & remove all areas of loose debris / loose body protection coatings but leave any sound areas alone unless you can see rust creeping under the surface

3 - Bilt Hamber Hydrate 80 is ideal for treating any rust borne areas (has already mentioned)

4 - Once dry I would apply good quality etching primer - (aerosol is fine) Bilt Hamber do one called 'Etch Weld' that is very good

5 - Again once dry apply a very high quality vehicle underbody wax - Dinitrol 4941 is what I use but Bilt Hamber supply a similar product called Dynax UB - both are available in a aerosol format if you do not have a air compressor and are ideal for smaller areas

6 - Here is a pic of the very same exercise that I undertook with a 30 year old BMW 535 recently - Dinitrol dry's to a OEM factory finish in appearance

[steveo3002]

look at what any pro restoration shop or decent bodyshop uses , its all epoxy these days , also what they paint marine stuff with so someone must think it protect in salty water

no need to stay in the past , do your own research you wont find many folk moaning about epoxys

i could write a book about being stuck in the past , could call it "i say what i like and like what i say" , don't mean I'm right though

[Deleted]

Oct 16, 2019 8:03:07 GMT steveo3002 said:

look at what any pro restoration shop or decent bodyshop uses , its all epoxy these days , also what they paint marine stuff with so someone must think it protect in salty water

no need to stay in the past , do your own research you wont find many folk moaning about epoxys

i could write a book about being stuck in the past , could call it "i say what i like and like what i say" , don't mean I'm right though

I am a professional restoration company - have been for a long time with a equally long list of accolades - you wont find many restoration companies / bodyshops that don't know what they are doing slapping hard setting epoxy coatings on the underside of vehicles nor will you find any new vehicle manufacture doing likewise - compounds used on underbody structures need to be of a flexible / rubberised coating that deflect impact but also flex with the vehicles structure whilst expanding & contacting to the many operating temperature cycles that vehicles operate in

Hard setting compounds that are manufactured for marine & industrial use do not hold the same impact resistance they tend to crack on impact hence letting moisture through - quite often in a capillary method and allowing rust to ingress under the coating and you will not find out until its too late

You will however find epoxy primers used in abundance within the motor industry / motor trade and very good they are too but epoxy mastics & epoxy primers are two very different products

Nothing to do with staying in the past but is a fundamental matter of using the right products for the right job

[Midland Performance & Retro]

Epoxy mastic is only any good for bodging up a small area as a short term fix if you don't want to go to the extent of doing the job properly, or masking over the problem.

Every professional worth their salt will know that!

If we had a complete underside of car in bare metal, which had been treated for rust, the process with us would be, Zinc Prime, repair, seal, Epoxy Prime (smaller repair areas more likely etch). Polyester Primer if the finish required warrants it, then top coat with a rubberised coating. Then Cavity wax and glaze with a clear wax. That's about as thorough and effective as you can get with what's available on the market today for me.

[redblock]

Oct 15, 2019 21:01:08 GMT @grumpynorthener said:

Ok has many know I run a small independent restoration business so a few things for you consider:

1 - POR 15 or hard setting epoxy mastics don't get a look in at my workshop and never will - they are useless and I can write a book on why - but I don't have the will or the time - you need something that will move - expand & contract with the vehicles structure & that will to a certain extent self heal when impacted - that's why vehicle manufactures use flexible compounds & sealers on the underside of vehicles when in production

2 - Degrease the area thoroughly with panel wipe & remove all areas of loose debris / loose body protection coatings but leave any sound areas alone unless you can see rust creeping under the surface

3 - Bilt Hamber Hydrate 80 is ideal for treating any rust borne areas (has already mentioned)

4 - Once dry I would apply good quality etching primer - (aerosol is fine) Bilt Hamber do one called 'Etch Weld' that is very good

5 - Again once dry apply a very high quality vehicle underbody wax - Dinitrol 4941 is what I use but Bilt Hamber supply a similar product called Dynax UB - both are available in a aerosol format if you do not have a air compressor and are ideal for smaller areas

6 - Here is a pic of the very same exercise that I undertook with a 30 year old BMW 535 recently - Dinitrol dry's to a OEM factory finish in appearance

That's really helpful, appreciate you taking the time to answer. I've found a similar wax-based undercoating from a US Distributor ( Woolwax, if you're familiar), and it looks based on the properties listed that it will behave much like that Dynax UB. Lanolin rather than bitumen base, non-rubberized. It dries, but not fully, and remains pliable. I'll also treat the rust with a converter and a nice etch primer before applying.

[Deleted]

Just looked up 'Woolwax' - I hade never heard of it before - you are correct in thinking that it will do the job - it carries strong recommendations / credentials and should be fine with what you have in mind - Chris

[kevins]

I also use rust coverter, epoxy primer, sealer then some sort of flexible stone chip followed by wax, A good tip I picked up is to use a light stone chip then cover with a black wax, as the wax becomes thin it is easy to spot and touch up.

[VW]

Oct 16, 2019 9:00:33 GMT @grumpynorthener said:

Oct 16, 2019 8:03:07 GMT steveo3002 said:

look at what any pro restoration shop or decent bodyshop uses , its all epoxy these days , also what they paint marine stuff with so someone must think it protect in salty water

no need to stay in the past , do your own research you wont find many folk moaning about epoxys

i could write a book about being stuck in the past , could call it "i say what i like and like what i say" , don't mean I'm right though

I am a professional restoration company - have been for a long time with a equally long list of accolades - you wont find many restoration companies / bodyshops that don't know what they are doing slapping hard setting epoxy coatings on the underside of vehicles nor will you find any new vehicle manufacture doing likewise - compounds used on underbody structures need to be of a flexible / rubberised coating that deflect impact but also flex with the vehicles structure whilst expanding & contacting to the many operating temperature cycles that vehicles operate in

Hard setting compounds that are manufactured for marine & industrial use do not hold the same impact resistance they tend to crack on impact hence letting moisture through - quite often in a capillary method and allowing rust to ingress under the coating and you will not find out until its too late

You will however find epoxy primers used in abundance within the motor industry / motor trade and very good they are too but epoxy mastics & epoxy primers are two very different products

Nothing to do with staying in the past but is a fundamental matter of using the right products for the right job

I'm still unfamiliar with epoxy lroducts although i do have some epoxy primer. Ive used it, but not on a vehicle (works very well on varnished wood though!)
Can you explain the difference between epoxy primer and epoxy mastic? I see very similar looking products using each name.
Thanks, always informative

[Deleted]

Oct 16, 2019 20:54:35 GMT VW said:

Oct 16, 2019 9:00:33 GMT @grumpynorthener said:

I'm still unfamiliar with epoxy lroducts although i do have some epoxy primer. Ive used it, but not on a vehicle (works very well on varnished wood though!)
Can you explain the difference between epoxy primer and epoxy mastic? I see very similar looking products using each name.
Thanks, always informative

Both are 2 pack products hence they are cured with a catalyst hardener - the primer is utilised for priming substrate areas that have been the subject of work - can be used on the vast majority of surfaces and adheres very well to bare metal - it can be worked / sanded & keyed once cured - epoxy mastics are more intended for industrial & marine purposes and provide a dense / thick protective coating over metals - for reasons already expressed within the thread I would not recommend it for automotive use

[VW]

Oct 17, 2019 7:57:16 GMT @grumpynorthener said:

Oct 16, 2019 20:54:35 GMT VW said:

I'm still unfamiliar with epoxy lroducts although i do have some epoxy primer. Ive used it, but not on a vehicle (works very well on varnished wood though!)
Can you explain the difference between epoxy primer and epoxy mastic? I see very similar looking products using each name.
Thanks, always informative

Both are 2 pack products hence they are cured with a catalyst hardener - the primer is utilised for priming substrate areas that have been the subject of work - can be used on the vast majority of surfaces and adheres very well to bare metal - it can be worked / sanded & keyed once cured - epoxy mastics are more intended for industrial & marine purposes and provide a dense / thick protective coating over metals - for reasons already expressed within the thread I would not recommend it for automotive use

Thanks, as i suspected, but it was unclear from searching for products.

[ant260z]

Oct 17, 2019 7:57:16 GMT @grumpynorthener said:

Oct 16, 2019 20:54:35 GMT VW said:

I'm still unfamiliar with epoxy lroducts although i do have some epoxy primer. Ive used it, but not on a vehicle (works very well on varnished wood though!)
Can you explain the difference between epoxy primer and epoxy mastic? I see very similar looking products using each name.
Thanks, always informative

Both are 2 pack products hence they are cured with a catalyst hardener - the primer is utilised for priming substrate areas that have been the subject of work - can be used on the vast majority of surfaces and adheres very well to bare metal - it can be worked / sanded & keyed once cured - epoxy mastics are more intended for industrial & marine purposes and provide a dense / thick protective coating over metals - for reasons already expressed within the thread I would not recommend it for automotive use

A lot of people these days seem to be advocating epoxy mastic as a better alternative to powder coating, mainly for suspension parts and similar, would you agree that's an acceptable use for the product, but not as you say on the underbody?

[Midland Performance & Retro]

Oct 18, 2019 7:18:45 GMT ant260z said:

Oct 17, 2019 7:57:16 GMT @grumpynorthener said:

Both are 2 pack products hence they are cured with a catalyst hardener - the primer is utilised for priming substrate areas that have been the subject of work - can be used on the vast majority of surfaces and adheres very well to bare metal - it can be worked / sanded & keyed once cured - epoxy mastics are more intended for industrial & marine purposes and provide a dense / thick protective coating over metals - for reasons already expressed within the thread I would not recommend it for automotive use

A lot of people these days seem to be advocating epoxy mastic as a better alternative to powder coating, mainly for suspension parts and similar, would you agree that's an acceptable use for the product, but not as you say on the underbody?

It's more suitable for "solid" components than the bodyshell, but it's still not what I'd use for a "proper" job personally. 

[Deleted]

Oct 18, 2019 7:18:45 GMT ant260z said:

Oct 17, 2019 7:57:16 GMT @grumpynorthener said:

Both are 2 pack products hence they are cured with a catalyst hardener - the primer is utilised for priming substrate areas that have been the subject of work - can be used on the vast majority of surfaces and adheres very well to bare metal - it can be worked / sanded & keyed once cured - epoxy mastics are more intended for industrial & marine purposes and provide a dense / thick protective coating over metals - for reasons already expressed within the thread I would not recommend it for automotive use

A lot of people these days seem to be advocating epoxy mastic as a better alternative to powder coating, mainly for suspension parts and similar, would you agree that's an acceptable use for the product, but not as you say on the underbody?

Personally I would not be recommending either epoxy mastics or powder coating on suspension components - both treatments form a hard thick coating over the substrate surfaces - However from my experience they are not that impact resistant and has a result tend to chip / damage easily - its simply not has durable has some vehicle owners are led to believe - I would recommend a good etching primer followed by a epoxy primer then a good quality colour coat - further protection can be added with a clear / transparent underbody wax once the components are refitted - there are specialist underbody component coatings in the market too such has 'Chassis Black' by Frost Restoration Products plus all major automotive paint manufactures also carry a range of products for this type of application.

[VW]

Powder coat shouldnt chip, but i hear it often does.

One of my first jobs involved a bit of powder coating. Nothing exciting, chair frames and stuff like that.
First one off the line was beaten with a hammer, mashing the tube up. If it chipped or flaked off the batch didnt pass inspection.
I wouldnt use it under a car though but purely because it seems to be hard to find anyone decent these days. It was 25 years ago i was doing it.
I have had a motorbike frame or two done. Alot of folk don't like it on bike frames but i found a decent firm (gone now) and they did a great job. Not too thick, properly prepped, great finish, properly stuck on!

[dodgerover]

Oct 18, 2019 12:32:52 GMT VW said:

Powder coat shouldnt chip, but i hear it often does.

It never used to chip, if it is doing now then something has changed. Fire was the only way I have found to remove it in the past other than grinder action.

[VW]

Oct 19, 2019 9:37:09 GMT dodgerover said:

Oct 18, 2019 12:32:52 GMT VW said:

Powder coat shouldnt chip, but i hear it often does.

It never used to chip, if it is doing now then something has changed. Fire was the only way I have found to remove it in the past other than grinder action.

As so many people seem to dislike it due to chipping i can only assume its either not true, powder coat tech has got worse, or no one is prepping or applying it right.
I think maybe folk are getting it done at places that just don't care and normally coat cheap garden furniture. This then leads to a general bad reputation of powder coat.