filling control joints with fiberglass

[thunderstruck]

Hey guys. Im about to coat a 40 day old concrete floor and am tossing around an idea for filling control joints, but I wanted some other peoples opinions on it. The coating Ive decided on using is Rocksolid. I was going to fill the control joints using sikkaflex, then coat over the top of it, but Rocksolid told me that if I wanted to put the coating over the top of the filled joints, I should use their fast patch kit. ( 1 qt. $70). Id really prefer a somewhat seamless look, so Id really like to coat over the top of the joint rather than fill them after coating, but #1, I dont want to wait a week for the product to arrive, and #2, Im not excited about spending $150 to fill joints.
Ive read about some people filling joints with bondo. Having bodywork experience, I know that bondo is not a very hard product,as in you could take a screwdriver and easily scrape a gouge in it, but more than that, it will absorb water. Ive used a product very similar to bondo, but its fiberglass instead. It mixes with a hardener just like bondo, but its waterproof, cures quickly, and is much harder to damage than bondo. and I can easily get it at an auto parts store for about 50 bucks a gallon.
I assume it would be best to have a somewhat flexible filler, but if I end up getting a hairline crack in the joint, Im not sure itd be that noticeable, so Im not really concerned about it. Maybe thats a mistake. I also thought about filling the joint with fiberglass resin. No mat, just the resin.
So, those were my two thoughts, now let the opinions fly!

 

[pmilin]

Are they saw cut joints? I just use a real good concrete caulk. Normally the joints are pretty deep so first install a backer rod. Home depot has it. then lay the caulking in the joint and use a scraper blade to smooth it flush with surface.

 

[thunderstruck]

Its my understanding from rocksolid that they dont recommend applying their coating over anything but their fastpatch. Im assuming this is because if its flexible, the coating could crack or come up? Im just guessing. I assumed Id have to have something hard in there?

 

[Edger]

Just a note that Sikaflex will shrink in winter leaving a dip in the joints and will squash in summer leaving a protruding round hump. After a season the epoxy will craze crack on top of the joints.