question on paint removal before welding

Burnt paint leaves a residue that contaminates the weld.

This also depends on where you are. In the USA we must remove all lead based paint, this means testing for lead on each project. It is sometimes easier and less time consuming, not to mention less expensive, to just call it lead based and remove it.

Secondly, as mentioned already, the residue.

Next, well have you smelled burnt paint?

_{And I thought you are alien}

Any welding procedure says (or should say) - remove all rust, scales and foreign materials from the welding and Heat Affected Zones (HAZ)

It has got along with the above problems (paint ash in the welds) other better ways of weakening the weld (porosity, Hydrogen embrittlement etc)

As you must know that the weld area should be free of gaseous Hydrogen - so much so that nothing that can generate the Hydrogen should be around - and water/ hydrocarbon-et al - may crack and give out h2 which will be greedily accepted by the molten steel.