You are not trying to melt the ramp, just the microscopic hills and valleys caused by the sanding, which when smoothed out returns the ramp to crystal clear.  You can flame polish with decals left on but I find every thing happens a little quicker, so you need to be careful, I think the reason for this is the decal prevents the heat from  escaping a little and the ramp therefore gets hotter. 

The first pic is after a few strokes across with the flame.
The second pic is the finished result of the first ramp