I am in the process of repairing my VIS fiberglass rear bumper. It developed some large stress cracks due to an impact.

I went to my local body shop, and they sold me some U-POL liquid gold glazing putty to fill in the stress cracks with. The area is pretty large...maybe 2 feet by 2 feet or so. I sanded the fiberglass bumper down with 80 grit pads with a DA sander in the effected area.

I cleaned up real good and applied the glaze with a spreader.

Everything is all dried up now and really have only one question:

How do I know that the glaze made it into the cracks and how do I know that by sanding down with 80 grit once again with the DA sander, I won't be sanding too far and revealing the cracks again. I sanded in a sample area, and I can see the cracks again. That kind of freaked me out.

Will the cracks be filled but visible? Is it possible to sand too far and if so, how do I know that I have sanded too far besides priming it and finding out the hard way?

I have never been good at checking a surface by hand. Everything always feels so smooth until I prime things then I find out how uneven things are.

Thanks

when you sand it there will be differant colors in it kind of you will be able to see them and that will show highs and lows and always use the palm of your hard to feel it if your using your hands i always if i can use a sanding block too even if i wrap the paper around it that way i know its flat vs my fingers you should be alright if you go beyond the spot that is cracking a bit that way you can taper it off slowly and you wont have to sand as far if you know what i mean

Thanks for the tips, especially the tip on using your hand when possible instead of fingertips. I thought about it more and think that the glaze is designed to work into the cracks, so if you sand further it can't matter. If you sand through the cracks you are simply sanding into a clean surface. Of course that your surface may not be perfectly smooth, but that is the least of my worries as long as it can't be seen by eye. Add some high fill primer afterwards and I bet it all looks good.

I hope I don't come back crying!

yeah i bet if you use a self etching primer afterwards it should fill in small amounts just dont go to crazy when you sand it i would say ive never used what your talking about i have used just fiberglass and went beyond the cracked areas and leveled it out as i went and it looked good