TheSamba.com :: View topic - Prepped Epoxy Primer with 400 Grit for Fiberglass...Wrong?

Thu Sep 24, 2015 8:17 am

Hey all,
I got my car sandblasted and epoxy primed and I need to cover up butt weld marks on my quarter panels. I scuffed up all my panels with 400 grit as the paint indicated and then started laying in fiberglass. Then I got worried. Some websites say you need to go as low as 80 grit to prep the substrate for fiberglass. Any input? Do I need to take the glass off and re scuff at 80 grit?
Thanks!

Thu Sep 24, 2015 1:49 pm

don't know if you need to go down to 80, but 400 is WAY too smooth for fiberglass filler to bite. Maybe 120.

Thu Sep 24, 2015 3:15 pm

I agree. 400 is probably too fine. You might just grab a pick and see if you can get it to lift easily. If it feels like it is holding good, leave it alone. If it seems to peel off, take it all out. I would use 80 grit to get a good mechanical bond.

And I hope you dont really mean fiberglass. Im hoping your meaning fiberglass reinforced filler.

Thu Sep 24, 2015 4:41 pm

Depending on the type of epoxy and the amount of time after applying you may not need to sand or scuff before filler at all as it can form a chemical bond. Also I hope you meant filler, not fiberglass.

Fri Sep 25, 2015 6:24 am

Quote: And I hope you dont really mean fiberglass. Im hoping your meaning fiberglass reinforced filler.

Yeah..me too.

The "toothier' the substrate the better for all fillers.

Fri Sep 25, 2015 6:44 am

Wait... I can't just use fiberglass? :shock: I'll be right back! 8)

Kidding, kidding. On panels that had bigger waves or butt weld marks, I've been using tiger hair to fill the big waves that I couldn't hammer out with a hammer and dolly. Then I moved in with the everglass to get it to the point where I almost feel like I don't need bondo.

That's when I realized that I screwed up putting the tiger hair in over not well sanded panels so I need to take that back off.

I'll bondo the panels afterward and get as close as I can with bondo and block it with guide coat before primer so I don't have to shoot primer more than once [-o<

Thanks for the help guys, I pretty much knew the answer when I posted the question but it always sucks to do work twice, even when it's the right decision.

Fri Sep 25, 2015 11:37 am

When you say you moved "in with everglass" what are you referring to? Are you using matt/resin over the tiger hair to build thickness?

Fri Sep 25, 2015 7:08 pm

I'm sanding down the tiger hair with a jitterbug to get the shape close to where I need it but in cases where there's minor lows in the tiger hair it seems like it would be best to follow with everglass because it's easier to work with and seems to hold a thinner layer better. Then I sand that back down and check again for highs and lows.
I've gotten that far on one panel that I've prepped properly. Now it feels really nice but I know there's some lows even the everglass might not be easy to work into, being shallow and mellow.
A skim coat of bondo over that will get me even even closer so I have to worry less about finding issues after a couple coats of high build primer.
Sound good?

Sat Sep 26, 2015 7:16 am

Ok...got ya now.

So your using the long strand reinforced filler, followed by short strand filler, followed by regular filler (don't use Bondo brand) right?

With all those fillers I would be worried about total thickness. You really shouldn't ne over 1/4" in total thickness.

Fri Oct 02, 2015 6:00 pm

no , dont use bondo , you want to use metal glaze ! its a lot creamier , and sands a lot better !

Sat Oct 03, 2015 5:24 am

I wouldnt use glaze for that. Just a decent filler. I think your right on track. In fact it wouldnt hurt to skim the whole panel. Spray your guide coat, then knock it back down. Use a nice cross hash pattern when you sand and you will have a good shape and all the low spots will be filled. Guide coat again and back over it with some 120-220 grit and then a nice poly high build primer. Guide coat again, one more time nice and easy with some 220. Then either right to 400 wet, or a coat or 2 of a final primer then 400 wet. Sealer is your preferance.