Yes you can. You should always check for compatibility of products, but generally an epoxy coating can be top-coated.
Link to Sherwinn Williams Macro-poxy with top-coat recommendations:
http://www.paintdocs.com/webmsds/webPDF.jsp?SITEID=STORECAT&doctype=PDS&lang=E&prodno=B58W610
take a light sand paper first and scratch the poly and then put the putty on make sure it is a color match and re-polyurethane
Sometimes people do this to get an antique look, I can't think of any other reason to do this.
Yes you can. Lightly sand the shellaced surface to remove any gloss, then coat with the polyurethane.
Varnish is unlikely to stick to polyurethane; it's designed to soak into porous wood, not to chemically bond to a compound like polyurethane.
Yes, you can
NO
A lot of resins are used in varnishes. Printer's varnish uses copal resin, polyurethane varnish uses polyurethane, amber varnish contains amber.
Bleach
I would use an exterior polyurethane. It is going to hold up better and be more flexible than a varnish. Polyurethane has taken over the varnish market--it's hard to find any other kind. There is a "spar polyurethane," which is what you should use on the item in question.
Yes, you can put polyurethane paint over eggshell in latex or eggshell in oil.
Can you put stain in polyurethane to darken the color?
A lot of resins are used in varnishes. Printer's varnish uses copal resin, polyurethane varnish uses polyurethane, amber varnish contains amber.
No, it isn't.
A lot of resins are used in varnishes. Printer's varnish uses copal resin, polyurethane varnish uses polyurethane, amber varnish contains amber.
Bleach
I would use an exterior polyurethane. It is going to hold up better and be more flexible than a varnish. Polyurethane has taken over the varnish market--it's hard to find any other kind. There is a "spar polyurethane," which is what you should use on the item in question.
Yes, you can put polyurethane paint over eggshell in latex or eggshell in oil.
Can you put stain in polyurethane to darken the color?
Varnish is not made of nylon. There are a number of different formulas for varnish, but the most popular kind is made from polyurethane.
Polyurethane becomes sticky because it takes an hour or longer for the oxygen-induce crosslinking to occur. Polyurethane is a varnish used on wood.
by covering it by cove (alternate answer) There is more than one possible sealant to use, but I would recommend painting it with polyurethane varnish.
Yes
If a polyurethane varnish (not lacquer) contained linseed oil, it COULD have lead in it - lead is a drying agent for that oil. It was mostly used in paints as a pigment. Varnish has no pigment and it dries by chemical reaction, so most polyurethane varnishes don't need lead.