Originally Posted by durwood
Tack cloth varnish used to be available to make tack rags but I havn't had to buy any for years as its not suitable to tack off some surfaces.
Spar varnish usually is synthetic enamel based so it can leave a trace of this on the surtface 9as can other mixtures)which you are going to paint. A lot of the new paints react with it and spoil you paint finish. There are special ones made for the auto trade usually because 2 pack finishes react. Often you have to get speciafic brands of tack cloth for particular brands of paint to avoid the problem. Try the tack rag on a testsurface before you wipe over and paint something you have just spent hours getting prepared. Anything with linseed oil it it would only be suitable for turps thinned paints.