Just stay away from anything containing even traces of lead. If that seems like too much, check your Yellow Pages for retinners in your area. A disappearing trade just like buggywhip makers. The copper surface should be brightly polished with very fine emery cloth, no corrosion remaining. Then apply the fluxpaste or liquid with a brush, heat the copper with the torch and when the sheen changes from bright to dull, apply the tin to the copper, not directly in the flame, until all the surface is covered with a good coat.
Tin can stand quite a bit of heat before it melts or evaporates.
A disappearing trade just like buggywhip makers.