Homebrew PCBs

Im getting ready to order some double sided FR-4 from abcfab and have
a question.

Right now I do not plan on making very many double sided boards. Its
more difficult to find single sided pcbs and was wonder how I could
cover the top side without putting a resist layer on it before I etch.
Is there a cheap paint that I could spray on it and remove after etching. 

Any other advantages or disadvantages to having double sided vs single
sided pcbs when you are only using one side.

Regards
Wag