Doc, it's the cost of the equipment (pressure pot rig) and the skill required to do it right with the "real stuff."
The approach I used is for non-surgeons
I'm happy with how it turned out, and would do it again. I'd encourage you to try it as well, unless pressure spray equipment is something you'd want to invest in anyway. I sweated a bit before I cut loose with the spray can, imagining having to sand it all off and start over if I botched it. Nobody likes scut work, right? But it turned out fine. In the end it was a confidence-builder.
Hop-Along Aerodrome (12VA)
RV-6A - N30YD - flying since '98
RV-10 - N130YD reserved - under construction
donating monthly to the VAF - thanks, Doug