PCB Tinning to increase current
While DrFriedParts provided an answer to the question you asked, I feel I ought to respond to your premise, instead. Specifically, "i do know this decreases the resistance of the tracks and increases the amount of current it can handle", while technically true, is not a reason to specify HASL. Let us consider. A 1 oz copper trace has a thickness of 1.37 mils, and a bulk resistivity of ~0.017 uohm-meter. A typical HASL layer is 0.1 to 0.3 mils, and the bulk resistivity is ~0.17 uohm-meter. So HASL will add about 15% to the cross-sectional area of the trace, but with a resistivity about 10 times greater than the trace provides. The result of this is that the resistance of the trace is reduced less than 2%, and the current-carrying capacity is increased by the same amount.

Trust me, if you absolutely must have the extra 2% you are doomed.

HASL is a perfectly reasonable finish, but is does not do anything noticeable to your current-carrying capacity.