Mr. Surfacer - what is it?

I had the exact same question. I purchased one each i a spray can. I shot it on some sample pieces. It wont cover up panel lines. It does cover up imperfections in the plastic and such. One thing I will tell you is that it will cover up detail. I cast a small piece with some latch detail on it in 1/48 scale. I lost the sharpness of the detail after I sprayed it.

At this point I see using it on seems that I've filled where I don't have to worry about losing detail.
Dave

Mr Surfacer is basically the ultimate in primers. Treat it as you would any other rattle can paint. The most significant difference is that no matter how poorly you spray it (short of huge runs and sags) it will dry perfectly and shrink down to give a nice even finish without obliterating any detail. That's the key difference from paint, it shrinks down without obliterating details.

Once dry, give it a light sanding to ensure it's smooth and you're done.

In the bottles, you can use the 500 as a liquid filler for small seams and gaps. Wipe or scrub off any excess with a Q-tip dipped in isopropanol (or similar acrylic thinner).

BTW it's lacquer based so if you ever get the bottles and wish to airbrush, thin it with lacquer thinner AND don't forget to clean the brush when finished.