I hate sand-thrus!

K

KevinH

Member
How about some helpful info on avoiding sand-thrus! I paint mostly motorcycles and seem to always have some kind of sand-thru somewhere along the line. Anyway I thought on the old web forum there was some info on epoxy adhesion to different grit on bare metal. I put 2 coats of epoxy on a fender and blocked with 80 grit, came back in about 3 days later and laid down 3 coats of epoxy with a flash time of 30 minutes between each coat. So after a day I decide Ill wetsand with 400, let set and this weekend spray 1 epxy sealer coat and go to base. Anyway while 400 I made a sand-thru!!!! So should I scuff that area with 80 and reshoot or what???

Dilema 2, Have a outer fairing (Harley) 180 grit and let it set out in the sun for awhile. Sprayed 2 coats of epoxy, 10% reduced. Laid down pretty good. Went to 600 grit but guess what, SANDTHRU, So now what? Will the epoxy stick to a 600 on fiberglass(gelcoat)?

Maybe I just needed to vent. And having to sand around those front fender rivets. UUUGGGHHH! What is some tips? Maybe since Im just using epoxy, I dont have enough mil thickness to start with?? THANKS!
 
kevin, its no big deal really, just mix up some epoxy and reduce as a sealer. shoot on 1 coat just before you base it. do yourself a favor too and grind the rivets down and weld those things in. only takes 10 minutes to do and some quick bodywork in those areas. i do it and dont even charge people for it because it actually saves me from the headache of trying to work around them. i always tell people i will throw that in as an extra just for them, to make them feel good......shhhh, dont tell i do it anyway. in the end the time equals out, the job comes out cleaner and the customer is happier.
 
I agree on the rivets! I just welded up the 8 on a front fender and 4 on a rear fender, nice not having to scuff/sand around them, also better for masking flames and graphics. I've noticed a few front fenders with rust problems between the brace/reinforcement and fender, also along the front edge on trim models, if you knock the rivets out and remove the brace it's easy to clean that area up and deal with any corrosion. Epoxy and seam sealer stops the start of any corrosion again.
 
Correct me if I'm wrong but after the 2 coats of epoxy wouldn't it be better to apply a high build or poly primer to smooth things out? Seems like building with epoxy would be more time consuming.
 
I have experimented with building with epoxy. Keep in mind, the people doing this are waiting a good while in between coats, and many let the car sit for a week or so before blocking. I have tried the one coat at a time method that you mentioned before Shine, but I have a hard time sanding even one coat of epoxy the next day. I'll bet it is the difference of TX vs PA weather.
 
i have had no problems with it. shoot in the morning then block the next day. i do not use reducer at all. but i also wet sand everything. i detest dry sanding always have. makes a god awful mess to deal with. when i'm done blocking the car is clean and ready to go. i cant figure out how guys are sanding through 2 coats of epoxy or 3 or 4 coats of clear.
 
there are certain plastic parts i do here that i use the epoxy to build on because it stays flexible and has next to no shrinkage. its a leathery texture i fill in. i have done it with 2k before and it just shrinks to bad. it requires 5 coats to fill so that i dont burn through when im sanding. i give about 30 minutes between each coat and do all 5 coats in one session. i let it sit about 3 days then sand it and the stuff sands beautiful. 2 coats i have no problem sanding after 24 hours. it takes a little more effort to work it down but i think it actually clogs the paper less than the 2k.
 
i block with epoxy too. if i have to sand a fender with rivets, i cover the rivets and edges with tape as well as having epoxy ready in case i burn through...
 
All good info! THANKS! So what is the finest grit that epoxy would adhere to? Metal ???? Fiberglass and plastic???? Just wondering with a 400 grit sand thru either gelcoat or metal, Im guessing with JimC reply that it will be ok with 1 reduced sealer coat. So what is the minimum mil that is recommended before base? THANKS!
 
Small sand throughs, dime sized, quarter sized....I don't worry about texturing the bare metal on these small spots before applying more epoxy but if you're worried just scratch up the little spots with some 180, it'll be fine.
 
i too dont like the speed of epoxy.. no problem for locking down the metal..but its a waste to wait around for it. especially when it comes to bike parts.. 3 or 4 days of drying i could have had the whole bike done.. gotten paid and customer is happy and riding.. people dont care to wait too long for a bike paint job.. its not a 2 year resto.. plus time is money.
 
3-4 days is not the wait time needed for normal epoxy use. normally i will put 2k over it within an hour or two. if its a reduced sealer coat i wait 10 minutes and put my base right down. unless i have to hit a small spot i personally use epoxy on everything. never etch primer.
 
Jim C;7252 said:
3-4 days is not the wait time needed for normal epoxy use. normally i will put 2k over it within an hour or two. if its a reduced sealer coat i wait 10 minutes and put my base right down. unless i have to hit a small spot i personally use epoxy on everything. never etch primer.
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ya but two primer sessions with it will be... u know what i mean jim.. highend resto type work makes no sense on bike parts..
 
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