I have had some success with the tin plating product sold by Rapid Online but it requires the copper surface to be extremely clean or results are patchy. Once made up the solution last for about six months in a well sealed plastic container (supplied).
Regards
Bob
Bob F Burns G3OOU
C Eng, FIET, MSE
G-QRP 6907
Secretary of Crystal Palace Radio & Electronics Club: www.g3oou.co.uk
Technical web site: www.qsl.net/g3oou
@BobFBurns on Twitter: http://twitter.com/BobFBurns