i think it has to do with a number of things
technique, action, strings,
on technique, instead of grabbing the strings, like if you were trying to pull a piece of rope, use only your fingertips to barely grab the strings, and just let the string roll off your fingertips,
depending on the style of slap your playing, your action will be your best friend, if your playing fast psychobilly stuff, the lower the better IMO if its some slower rockabilly stuff, then you can have higher action without struggling, and ripping your fingers to shreds, and like i said before, let the string roll off your finger tips if your actually grabbing the strings your letting more meat get pulled around (that sounds funny

)
and strings, I wouldnt play any steels that werent a solo set, I play fast psychobilly most of the time and it would be to difficult for me, I think you should be able to slap just fine on any strings though, nylons, guts steels that were mainly used for slap playing, I noticed roundwound strings give more potential for blisters
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