Reddit mentions of DEFY 1 Gallon Wood Brightener

This advice assumes that the grey color comes from oxidation (usually sun/weather exposure like a grey-ed fence/deck). You can certainly sand this coloration away (and there is some merit to that as you will end up with a nice smooth finish) however, if the oxidation is deep, you will be sanding away a fair amount of material and it will require a lot of effort. Enter oxalic acid. Oxcalic acid is used to "bleach" wood, and is the main ingredient in wood brighteners. Depending on where you live, most any hardware store will sell a similar product over in the paint section. Often labeled as deck/fence restorer. For instance, Home Depot sells Behr All-In-One wood cleaner. The stuff is awesome, just clean/debride the wood with a stiff bristled scrub brush, wet the surface, brush on the cleaner (can dilute 1:1 with water), let sit 10-15 min (keeping wet), and rinse. It will take the grey color from the wood like magic. After that you can sand the surface smooth (if desired) with much less effort than trying to sand through the discoloration. If there is any finish left on the wood, you will want to remove that as well before refinishing.

Also, not to contradict /u/dragon34, but the advice he got about not sanding between coats of poly isn't particularly great. Yes, re-coating quickly will allow the second coat to adhere without sanding, but it will not result in the best finish. At the very least, you should denib the previous coat before moving to the next. Otherwise you will end up with bumps from dust, insect, uneven application, etc. being trapped between the layers. I usually just buff the previous coat with some 0000 steel wool until burnished smooth, and then go over it with a microfiber cloth that has been dampened with denature alcohol. If you are putting in all the effort to restore, prep, and refinish the piece, it is worth spending an extra 10 minutes prepping properly between coats.