When varnishing an instrument it is nice to seal off the f-holes. This stops accidental drips getting inside, and it stops wood dust and such coming from inside your instrument and sticking to the wet varnish. The most effective way to do this is to use blue masking tape on the inside of the ffs, before closing the box, just the way that Strad did it*. Incase you don’t remember to do this until after you have closed the box, here’s another way to seal the ffs using foam sheet.
I use 3mm, dense foam sheet from the craft store. If you can get blue it mimics the masking tape very well.
Press the foam onto the f-hole and it will leave a clear impression. Cut along the lines at the ends of the wings but everywhere else cut about half a millimeter outside the line.
Wedge the foam into the f-hole making sure it ends up below the surface of the top.
* No he didn’t. Most experts believe he used the old fashioned tan-colored masking tape