Penetrating Epoxy Sealer

You would need to soak the uncured are with a solvent like denatured alcohol or acetone. You'll need to wait until the solvent fully flashes off then recoat the entire surface with Penetrating Epoxy. 

Generally speaking, epoxy tends to yellow or chalk when exposed to direct sun light for extended periods of time, which is why we recommend protecting epoxies, MAS being no exception, with a UV protectant varnish or paint for exterior applications.  All epoxy resin systems are not created equal. There are some UV resistant epoxy laminating systems out there that are slower to yellow over time, but I don’t know of a system that is 100% UV resistant. 

You want to try to remove the soft of sticky portions. Use denatured alcohol or acetone and check out the troubleshooting section at the top of the page there's an article that explains your exact situation. The article is titled  "How to remove soft or sticky resin"

You would want to use PES as the seal coat. Applying epoxy over poly can cause delamination issues. 

Unfortunately the washcoat creates a layer that can reduce the effectiveness of PES. 

You could certainly use it for that, you'd need 2-4 coats or until you reach uniform coverage. Keep in mind epoxy degrades in UV light so you will to paint it or cover it with a high quality spar varnish for UV protection. 

Typically you would want to use a water-based stain when using epoxy. If the oil-based stain is already applied, you will need to wait for it fully dry (check with the stain manufacturer for a timetable) then you can apply a thin seal coat. You may experience orange peel or fish eyes in the seal coat, if so, wait for it to cure, sand to knock down any deviations, liberally wipe the surface with denature alcohol or acetone then apply another seal coat. 

It can be used for that application

You could use penetrating epoxy to seal them. Depending on the volume you could use Deep Pour to fill them, the smaller the volume the longer the epoxy will take to cure though. Oil top dressings can be applied to epoxy 

Acetone and Denatured alcohol are solvents for epoxy resin, you would mix part A and B together then add the solvent. Environmentals like air temp, material temp and humidity will play a factor in the cure time. The coverage is entirely depending on the porosity of the material you are applying it to. Absorption on punky wood will be much greater and will require more material then applications on closed grain wood or dense hardwoods.   

It depends on the air temperature, the warmer it is the faster it will setup. You would likely need around 24 hours before it's hardened to a point it can be turned over. 

If you can no longer make a fingernail impression in the PES then you will need to scuff it up with sandpaper then wipe it down with denatured alcohol or acetone. If its still soft enough to dent with a fingernail, you can pour the deep pour without sanding. 

Yes it can be thinned with acetone or denatured alcohol up to 50%. 

Penetrating Epoxy Technical Data