Spar (oil base) won't dry on bar top and it has been 4 days so far.
Can I switch to water based Poly?
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Can I switch to water based Poly?
Not without first removing the oil-based varnish. It's not about any chemical incompatibility, even after thorough 'drying' of an oil-based varnish it's easy to get beading of a waterbased finish applied on top.
It is possible the spar won't ever dry properly because something has gone wrong, but generally with this sort of thing patience is rewarded and the varnish will dry eventually, although in extreme cases it can take two weeks or so.
If you can't wait for some reason your best option might be to strip the existing varnish off, clean the wood down with an appropriate solvent and then give it a light sanding once it has dried. Then start again.
With the spar still being tacky you might just be able to use mineral spirits/white spirit to dissolve it, but if it has partially cured it may require something stronger, i.e. a reputable chemical stripper.
Graphus
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It is possible the spar won't ever dry properly Do you mean environmental factors? Or perhaps something else? – Matt May 24 '16 at 11:56
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Varnish has a limited shelf life, especially after being opened. It's possible this batch is simply too old to use. In that case, yes, clean up and try with a new can. – keshlam May 24 '16 at 12:32
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@Matt, I'm not sure if it's possible for local factors to permanently affect the drying of an oil-based varnish. If it's too cold and/or too damp drying can be retarded by a huge amount, the varnish will seemingly never dry. If this is the cause however a short stint in a dry, warm environment is enough to kickstart proper drying/curing and sometimes within hours the varnish will begin to firm up, although full curing will take a couple of days. But the varnish itself can be the problem, not sure how, and if so no matter the conditions it won't dry without remaining tacky or soft. – Graphus May 25 '16 at 13:23