When polishing out deeper scratches or marring to paint or GRP a PTG (Paint Thickness Gauge) must be used. These come in two types. One measures the combined thickness of the paint and lacquer. Modern paints are lacquer over base coat and only the lacquer should be polished. If you polish through the lacquer the surface will require re painting. The PTG I have identifies the individual thicknesses of the lacquer and the base. This allows me to see exactly how much material I have . As we polish the lacquer of course becomes thinner so by using a PTG we eliminate the threat or danger of burning through the all important lacquer coat. Top end gauge like I use also identifies the thickness of gel court over a fibreglass weave. The gauge I use costs nearly £1500 but a cheap one that measures just the combined thickness can be bought for around 200 quid. A top quality decal is essential. You can buy 3 5 0r 7 year qualities which is the length of time the decal is guaranteed NOT to fade. High numbers also means better quality material is used.