Which is best "paint-on" etching resist?

I've been etching brass shim. I was wondering if anyone can suggest a good substance to use as a resist on the 'other' side of the brass (opposite the
mask). I've been using clear spray enamel but I think that it must be leaving pin-holes as the opposite side always comes out slightly pitted.
Someone mentioned aerosol paint that was oil-based - can anyone suggest a good brand as I'm having trouble finding oil based spray paints.
Thanks, Steve
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