What is the best way to thin polyurethane?

Question

I recently installed a door for one of my old friends (yes, it is the one that I installed upside down without him noticing it, and now I have a bit more time to correct it and coat it with polyurethane).

The problem is, is that this man gave me some old polyurethane that his wife bought him for their last anniversary together before she died at her job testing bungee jumping cords, and as a result this polyurethane is quite aged and too thick to use for the door as it is. It's gluppy and has lumps in it. How can I thin it down to get rid of the gluppiness?

This man is insanely attatched to this polyurethane and insists that I use it on this door...although he hasn't noticed that the window is on the bottom yet.

Answer 1

try mineral spirits.

Answer 2

Right to the last sentence...lol Read the label