May I apply base coat over cured SP epoxy

3

356 Porsche

New Member
I am restoring a 1965 Porsche 356 SC and I have the body straight and flat. The car has three coats of SP epoxy and has been blocked with 180 grit paper. If I sand the car with 600 grit wet/dry paper will I have good adhesion when I spray my base coat. I would like to hear the pros and cons on this subject.
 
When I apply a sealer the surface is never as smooth as it is after sanding so my question is, will I get good results if I let the sealer dry and sand with 600 before applying the base?
 
If you want to do your procedure with epoxy, you have to do it quickly, within 48 hours after spraying. If you need a more relaxed timeline, spray the sealer, then spray one coat of base. Then you can sand with 600-800 at your leisure but don't sand the base all the way off. When I do this I like to use up base coat of the same brand that I have on the shelf, hopefully something close in color.

Waiting too long between epoxy and base is asking for trouble. If you don't like the sound of that, just get some 2K urethane primer, then you can use the more conventional process of waiting for the primer to cure, sand, than spray with no sealer.
 
Crashtech,
The car is going to painted a dark blue so I want to have a vary dark or black base under the paint. I have some 2K high build primer and would like use it, however, it is gray. How much (what percent) can I safely tint with black so I have a dark undercoat? I understand that if I do tint must be a urethane tinting base from a paint mixing system, is that correct?
 
I wouldn't tint a primer unless it is designed for it and has specific instructions on how to do it in the TDS. Usually the amount of toner allowed will not really get you very far, imo. But black covers very well, so there is little reason to require a similarly colored ground coat or primer. Many dark blues cover well also. A sprayout card should always be done to determine number of coats to achieve coverage. Sprayout cards can be obtained (usually for free) from the paint jobber.

If a black urethane primer is desired anyway, SPI makes a couple of different types and there is also black epoxy, of course.
 
Ok. I think I will purchase a gallon of the 2K black sealer and sand with 600 after it is dry. I want a dark undercoat so if there is a paint chip it will not show as much as a light colored undercoat will.

Thank you for all of your help!
 
356 Porsche said:
When I apply a sealer the surface is never as smooth as it is after sanding so my question is, will I get good results if I let the sealer dry and sand with 600 before applying the base?
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This is what I do. I’ve never had an issue with anything sticking to epoxy.
 
Wow I never realized epoxy was this finicky with adhesion. Guess I’ve been lucky. Always thought you could scuff and spray. I usually try and use a form of sealer if I can so I have a mechanical and chemical bond. But not gonna lie I’ve done the scuff and spray on the cheap stuff before. Hmm.
 
There are definitely different levels of adhesion. For maximum adhesion, epoxy on epoxy is the ultimate. 2K will stick to sanded epoxy, even cured, but not as well. I used 2K sealer over sanded factory epoxy for many years without issues, but I would bet it chipped easier than epoxy would have. I have seen guys scuff e coat and base right over it. That is a guaranteed failure. Base does not like cured epoxy. What makes epoxy great is the ability to be 100% moisture and solvent proof once cured. The slow drying properties leave a lot longer window for chemical adhesion than 2K, also.
 
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Done right, most things are a lot of work. Including doing restorations or general paint and body. I'm like 356 Porsche stated above, I don't really care for wet on wet sealer because it adds texture and the nature of wet on wet (urethane) inherently shrinks over time. Even wet sanded and buffed, I've had experiences over the last 30 plus years, that I've had to resand and buff because the sealer continued to shrink for weeks. I love SPI Epoxy, I think it is superior to anything on the market and superior to any automotive epoxy ever on the market. But it is epoxy and there are rules for dealing with epoxy. YOU MUST adhere to the minimum temperature requirements and YOU MUST know that it will crosslink so tight over time that there will nothing left but mechanical bond. So why use it Bartman?? There is nothing better for adhesion and corrosion protection, you just have to follow the rules.
 
Bartman,
Neither one of use like wet on wet or an unsmooth surface so how do you think I should proceed? Would it be a good idea to spray SP's 2K sealer over my completely cured epoxy (and sanded), let it dry sand it and them apply my base coat. I am always looking for the best job possible. This car will be shown and is worth a lot of money.
 
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