Tanky hit the nail on the head... it can become your best modeling friend!!!
It is nothing more than Future acrylic floor polish from the supermarket. It's about $4 a bottle and one lasts forever. Seals and protects paint, a must for panel lines and other weathering... There's an article about it on swannys webpage as well.
http://www.swannysmodels.com/TheCompleteFuture.html
oops had to put the right URL in.. there ya go!!!