I found the following information from the Yahoo Group Homebrew_PCBs.

In the September 1971 issue of "Ham Radio Magazine" Larry Hutchinson had an article "Practical Photofabrication of Printed-circuit boards". In that article he had an electroless tin-plating bath.

Formula

• 0.5g Stannous Chloride
•  2.0g Thiourea
•  3.0g Sulfamic Acid
• 100.0 mL Distilled water

To 100ml distilled water add 500 milligrams Stannous Chloride, 2 grams Thiourea, and 3 grams of Sulfamic acid. The chemicals are all powders so they keep but the bath is unstable, so only mix enough for one days job. All you need is enough to cover the board. Action takes 10 to 20 minutes. The article suggests a few mg of Alizarin or Alizarin Red S can be added as a brightner.