What you got from the auto parts store is probably polyester resin and the cloth is real thick. I've tried polyester resin but didn't like it because it wouldn't cure right sometimes.

For airplane use get glass cloth that is 3/4 or 1 oz. weight from the hobby shop or someplace like Sig or Tower. That stuff is very light, like silk. You cut it out a little bigger than the area you're doing, lay it on the surface and smooth it out.

Then you brush on either epoxy finishing resin(which is thin), regular epoxy thinned with acetone or even water-based polyurethane. You start from the middle and work towards the edges. After it hardens you trim or sand off the excess cloth, then put on another thin coat. After that hardens you can start priming and sanding.