Epoxy layer was soft after coat of poly

craig429

Member
Here is what I did. The shop was heated to 70* for a day before applying the first application of epoxy. Sunday, sprayed epoxy. Tuesday applied body filler in a couple spots and sanded the filler and had some sand throughs. Wednesday sprayed epoxy over the body filler and over the unsanded epoxy that was sprayed on Sunday. This application of epoxy was sprayed very heavy and had a couple runs, but not in the area of the problem. I put a standard 500watt shop light on it to heat it up to 95 for a couple hours, to help cure the heavy epoxy a little quicker. Friday sanded the runs out and sprayed poly. Saturday sanded the poly and in a spot sanded through to Wednesday’s epoxy. I noticed it was soft. I was able to scrape Wednesday’s epoxy off down to Sunday’s layer of epoxy with my finger nail The shop temperature was between 70 and 73 the whole time. I checked the soft epoxy on Sunday and it is no longer able to be scraped off with my finger nail.

Is it normal for epoxy to be soft even after waiting 2 days before applying poly primer? The layer of epoxy just under the poly is getting harder now that 4 days has passed since spraying the last application of epoxy.

Should I have waited longer before sanding the poly, knowing I will have sand throughs to the epoxy?


537CC7BE-C9D2-4F2A-A269-4FB8BD77A756.jpeg
 
Last edited:
That is similar to what will happen if you apply filler over epoxy after overnight dry and sand with a coarse sandpaper like 80#. It will rip the epoxy off the metal instead of feathering out. Putting it in the sun or just giving it another day to dry before sanding will give you the answer. I suspect it will sand normally, but the epoxy needs a full 48 hours before poly for that reason. If it stays soft for another day or 2, I would strip it off.
 
I think you may not know what your actual surface temps are. I would suggest getting an infrared thermometer, it will give you a good idea of the actual temperature of your parts. In our climate at this time of year, we can have the air temp almost uncomfortably warm for our taste, but the surface temps of the parts will be in the low 60's. Also, if the epoxy was applied heavy enough to cause runs, that rings a serious alarm bell as far as cure time. The good news is if the epoxy has not been exposed to low temperatures, it will still cure, but it will need quite a bit of heat and time to do so now, because it is insulated under a blanket of poly primer. Heat and time, and plenty of it, before you decide what to do.
 
Thank you for your reply's. The qtr extension was off the car when I sprayed it with epoxy and I sprayed it too heavy where it looked like fish eyes. I did an experiment and it failed, by spraying more on it to try and smooth it out instead of waiting for an hour or so for it to flash. That is when it ran.

I was sanding with 80 grit, and it did rip one layer of epoxy off the previous layer of epoxy. I will let it continue to cure at 70*, and will see how it sands.

I am using an IR thermometer. It takes a long time for the metal temperature to become the air temperature. And metal that is closer to the concrete floor is cooler than metal that is 4 feet off the floor. I have found out, the more days the shop is at 70*, the more stable the metal temperature is to the air temperature. The floor is a large heat sink..
 
You tested the epoxy, how?
You can never sand epoxy in an hour.
Yes, if temps are perfect the first few days, you cannot feather with 80, 180 yes the next day.
The test I give to shops when they are not sure the new helper mixed it right.
At 70 or higher in 24 hours, take a razor scraper, go to the center of a panel, and you usually will shave 3 to 6 inches off.
48 hours go to another panel, and you may shave an inch or 3 off.
72 hours you will gouge the epoxy and may see metal or may not.
Now I can say it was mixed correctly.
 
My play garage I keep 70- 24/7
And it's super-insulated; when I play with the temp gun, the hood will be 65, mid door 63, and the rocker about 60 rockers.
The same car has been in there a month and never changes.
 
Last edited:
I've had problems with poly primer on top of epoxy primer before. There is a general rule in painting that you should be careful putting fast drying material on top of slow drying material. Poly on epoxy is about as drastic as it ever gets. Nowadays if I am going to do that, I either wait long enough for the epoxy to be sandable, and thoroughly sand it prior to poly, or only use ONE coat of epoxy, then poly the next day.
 
Last winter working out of my garage I found that I had to get air temps up to around 80 degrees to get the metal temps to consistenly read 68 or more in various places. Setting up a small fan pointed straight up seems to help some as well. Personally being a tightwad I like getting it out into the sun whenever I can. Black epoxy even in the winter will heat up nicely plus the UV rays really speed things up. And it's free.:)
 
I am sorry for the confusion. I didn't sand the epoxy in an hour. I was sanding with 80 grit on the 1 day old poly that was on top of 3 day old epoxy that was on top of 5 day old epoxy. The experiment that I stated I tried, was I tried to see if I could spray more epoxy to smooth out the heavy coat of epoxy that looked like small fish eyes, but it sagged on me. Instead of spraying more epoxy right away, I should of not worried about the fish eye look and waited the 2 days to spray the poly. I knew I would not have been able to sand the fish eyes out of the epoxy for a day and I had extra epoxy in the gun to spray. Spraying the rest of the mixed material out of the gun is not always the best, it can cause problems.
 
Chris, In the winter when you put the car outside in the sun, do you spin the car around so each side gets direct sunlight?
Yes when I am able to. My current project, I made sure I can attach some small knobby tires to the body cart, so that I can move it more easily over my gravel driveway. Lots of times it's not viable with something so heat up the garage it is. Smaller parts, doors, hoods, fenders, etc I almost always put out in the sun. For me the lack of space dictates that I have to do everything in stages. So I try to save where I can.
 
Assuming it was mixed properly?
Ok it's driving me crazy, what part is that in your pic? For the life of me I can't put it together. o_O
 
Last edited:
I couldn't figure out how to rotate the picture. It is a picture of the RH quarter and quarter extension of a 69 Mustang. The bottom of the picture is the back of the car, the right side of the picture is the bottom of the quarter with a cutout for the side marker.
 
I've been using SPI Epoxy for years and on this forum for a number of years. These issue come in around this time of year and it's almost always temperature and time related. If you can't bake epoxy, you really need to keep it as warm as you can as long as you can.
 
I remember the first time I used epoxy....it was transtar. I had no idea the stuff was so temperamental to temperature. i sprayed it, turned the heat down to like 60 and went in, the next day sprayed poly and when blocking it the epoxy was soft as hell... kept that garage at like 80 for a week and then I ended up stripping the roof down :(
 
I couldn't figure out how to rotate the picture. It is a picture of the RH quarter and quarter extension of a 69 Mustang. The bottom of the picture is the back of the car, the right side of the picture is the bottom of the quarter with a cutout for the side marker.
:D I knew I recognized that curvy top part, but I never would have figured it out. My ocd had kicked in and I couldn't let it go. Thanks much.
 
I remember the first time I used epoxy....it was transtar. I had no idea the stuff was so temperamental to temperature. i sprayed it, turned the heat down to like 60 and went in, the next day sprayed poly and when blocking it the epoxy was soft as hell... kept that garage at like 80 for a week and then I ended up stripping the roof down :(
Yes indeed. Here is SC we are good to go spring through early fall. We have an "epoxy procedure" once the night time temps get below 55 degF. We're fortunate to have a baking booth. Our procedure is to mix the epoxy and put in the booth with the vehicle. We do a short bake cycle of 15 minutes at 120 to get everything including the epoxy warm. We've found that 15 minutes will get the metal temp to near 100 and the epoxy 85 deg or so. Then we switch to spray mode with the heat at 70. Once we have applied the required coats, we bake at 140 for an hour. Next day we go to work on top of the epoxy. Zero failures in many years of doing it this way.
 
Back
Top