Filler over SPI epoxy AFTER 30 days

MAKZ06

Newbie
I need some clarification on applying filler over SPI epoxy. I‘m only able to work on my projects typically every other month because we don‘t live full time where I have my vehicles.
I understand I have a 7 day window to apply filler over the epoxy without sanding or scuffing. However, I’m confused after reading some other posts regarding when filler can be applied over older SPI epoxy even after its been sanded.
I sprayed my last coat of Epoxy back in May. I’m once again block sanding the panels and have found additional low spots that need small amounts of filler. Can I apply filler directly over the epoxy which is freshly sanded? Or do I really have to shoot another fresh coat of epoxy before I can proceed? It’s so difficult and time consuming for me to get setup again to paint...
Another question. What will happen if I already applied filler (last year) over sanded epoxy that had been sprayed several months before? It’s now sandwiched in between the last coat of epoxy... Will it fail?
 
After 30 days, you must re-epoxy for proper adhesion.
Well that is unfortunate. So am I looking at potential catastrophic failure with what I had already filled or Is it more akin to applying filler over bare metal and is just not as good as to freshly sprayed epoxy?
what I’ve already done had been very thin layers applied to epoxy which had been sprayed many months or more prior,the old epoxy had first been sanded with 80grit, and then fresh epoxy went over that.
wow. I may shoot myself if I have to take it all the way back down to bare metal and start over.
 
I could be wrong. If so someone correct my thinking. I don't think Barry is saying it has to be taken down to bare metal again. Clean and sand the existing epoxy, then put new epoxy over the old epoxy and keep on trucking. I've done that several times and not had any problems. Like John Long said Barry would not give you any advice if it wasn't correct.
 
I could be wrong. If so someone correct my thinking. I don't think Barry is saying it has to be taken down to bare metal again. Clean and sand the existing epoxy, then put new epoxy over the old epoxy and keep on trucking. I've done that several times and not had any problems. Like John Long said Barry would not give you any advice if it wasn't correct.
Yeah, that I can do. However, I previously applied filler over sanded epoxy that was cured for several months.... I did not respray epoxy before applying the filler because I didn’t know that was necessary. Missed it somewhere in the documentation. I thought I just needed to sand the epoxy before reapplying filler or more epoxy if it was outside the recoat window...
 
I’ve done that too - as long as you sanded the epoxy with 80 grit you should be ok - filler likes 80 grit - BEST practice is fresh epoxy though
 
I’ve done that too - as long as you sanded the epoxy with 80 grit you should be ok - filler likes 80 grit - BEST practice is fresh epoxy though
That makes me feel better. Definitely sanded with 80 prior to applying the filler. I’ve been sick to my stomach thinking about it all day...
 
I believe what Barry was stating was that after 30 days you lose all chemical adhesion, and are then relying on mechanical adhesion only, which may not be sufficient. Sand an area where you applied filler over the old epoxy and see if it feathers out, or try to remove it with a razor blade. I believe it will be easy to tell if it is "stuck" or not.
 
It definitely feathered-out and appeared to be behaving as I expected. Most that I had applied was to cover imperfections around patch welded areas and was under 1/16” thick. I had carefully made sure all the low spots and surrounding epoxy had been covered good with 80 grit scratches. None of the filler lifted or left a transition lip you could feel after sanding. It even got sanded again months later before the next layer of epoxy was sprayed over it, and it maintained the feathered edge.
thanks for the advice. I know better now how to do it correctly.
 
I've been reading this thread about applying Bondo to old epoxy and have a couple of questions. I primed my engine compartment in June 2020 and like others, I assumed that I would be able to apply Bondo to the epoxy anytime after the 7 day window as long as I scuffed the epoxy first. But from reading this thread, it appears my assumptions are inaccurate. Additionally, if the epoxy had been sprayed 30 days or more prior, you must first sand it with 80 grit and then apply new epoxy for a new 7 day window. Then you can apply Bondo. There is though, a possible exception to this and that is, if you applied your Bondo to epoxy older that 30 days and sanded first with 80 grit you may be ok as long as you can feather out the Bondo. Do I have this right? Here is my own experience after reading this blog. I applied Bondo within the 7 day window to my engine compartment to smooth out some imperfections to all but one small 3" dent. So, yesterday I got out my die grinder with a 2" 80 grit pad on it and proceeded to scuff the small dent. Well, obviously it didn't scuff or sand the dented area, but it ground the epoxy off rapidly, leaving the surface mostly bare metal. I don't know what I was thinking when I grabbed the die grinder instead of a piece of 80 grit sandpaper. Anyway, my question is where do go from here. I don't have any epoxy left and this is the only little area I have to do before applying a topcoat. Can I apply the Bondo to the bare metal and then prime it with a urethane primer. I'm planning on using a single stage urethane topcoat to the engine compartment. Thanks, any help is appreciated.
 
Buy some more epoxy. You do not want to apply filler or topcoat over aged epoxy. Getting in a hurry here could cost you dearly. The epoxy does not have to be sanded with 80# to reapply epoxy, but it does need to be sanded with 180-320 and a red scotchbrite. Get some more and apply 1 coat to the area that needs filler. Wait at least overnight before applying filler. Finish sand filler with 180-320, then reapply epoxy to entire engine compartment and topcoat after overnight up to 7 days if unreduced. Finish sand with 320 if epoxy is reduced, and topcoat within 18 hrs. without sanding.
 
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Thanks for your reply. Can I just sand the existing epoxy and then topcoat with a single stage. Is there a scuff pad equivalent to 180-320 grit?

regards
 
I wouldn't. That epoxy is cured by now, and any adhesion you get will be mechanical only. Any spot you miss will not have good adhesion, and it is about impossible to sand every little corner in an engine bay. Spraying epoxy as a sealer would give great adhesion everywhere. You can buy quarts :) Red scotchbrite is equivalent to 320-400
 
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Another advantage to sanding the old epoxy is that if you find any other places you need to improve on, now is the time to do so. Nothing worse than getting color on it and wishing that you had taken the time and effort to fix just one more place.
 
I wouldn't. That epoxy is cured by now, and any adhesion you get will be mechanical only. Any spot you miss will not have good adhesion, and it is about impossible to sand every little corner in an engine bay. Spraying epoxy as a sealer would give great adhesion everywhere. You can buy quarts :) Red scotchbrite is equivalent to 320-400

Tex, thanks for your replies, I appreciate them.

If I had know that using SPI epoxy primer was so limiting and complicated I never would have used it. I'm not a painter and having to adhere to all these strict SPI epoxy requirements is a hassle and it also gets expensive because I have to hire someone to spray the epoxy.

You said "That epoxy is cured by now, and any adhesion you get will be mechanical only", how is this any different than applying Bondo to bare metal, that's been used very successful and has been the standard for 150 years.
I agree that applying Bondo over epoxy has it's advantages, but Bondo on bare metal has also preformed very well for a very long time. It seems, since the advent of epoxy primer, the thought of using anything different is taboo.

My original plan was to apply a satin black topcoat over scuffed epoxy but I've been advised on this blog that I shouldn't do that if I'm beyond the 7 day window. I do know several people who used epoxy primer who didn't apply a topcoat until "years" later and then only scuffed the epoxy prior to the topcoat and according to them never had a problem, granted they didn't use SPI epoxy. They did however use top quality epoxies like PPG dp 90 and other comparable epoxies.

With all that being said, I don't really want to mess up this 6 year and counting restoration. So, I may alter my plan of topcoating with a satin black single stage to just going with epoxy alone. I think this will resolve the issue with the dent that I inadvertently ground the epoxy off of and finally finishing the engine compartment. I've read on this blog that some have used epoxy alone, for various parts of there project, i.e. frames, undercarriages, and firewalls, but I have not heard of anyone using epoxy alone to do a complete engine compartment. You said in your last reply, that I would need to sand the existing epoxy with 180-320 and a red scotchbrite pad before topcoating. Can I just use the red scotchbrite pad for the entire engine compartment?
Please let me know where I'm going wrong and your thoughts on using epoxy alone for my engine compartment. Sorry for the length of this and thanks for the advice.
 
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Guys are just trying to help you get the best performance. You can just scuff it like any other epoxy if that’s what you want to do. It should perform as well as any other scuffed cured epoxy imho.

I’ve used epoxy alone on engine compartments. Works fine.

Don
 
With all that being said, I don't really want to mess up this 6 year and counting restoration.
There are a hundred different ways to do what you are doing. I am suggesting the way I know will give results from much trial and lots of error. It is true that bondo has been applied over bare metal millions of times. It is also true that more than likely, after a period of time, the metal will be rusted under it. Seen it hundreds of times. Applying filler over epoxy is not something new. I have a Camaro in the garage that was done in 1990, all DP 74 under any filler and 2K. No rust to this day. All products adhere differently to different substrates. Epoxy adheres better to cured epoxy than urethane SS. That is why I suggested reapplying epoxy. If you got every square inch sanded perfectly, you could shoot topcoat. Would it scratch or chip easier than if it was over fresh epoxy as a sealer? No doubt. SPI is the top quality epoxy. Never used anything comparable. The old DP was good, but you couldn't sand it like SPI. The DPLF and Shopline epoxies are complete garbage, IMO. There are rules and procedures for every product to get the best results. SPI is no different. Epoxy alone is a great option for an engine compartment. It is very chemical resistant and tough as nails once cured. Brake fluid won't even touch it. Try that with a SS urethane. A red scotchbrite will work for adhesion, but I would lightly sand it first just to knock down any trash or texture.
 
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