MrSurfacer what it is used for?

The 500 is thick and can be used to fill small gaps, pinholes, sink marks, and other imperfections. Apply it with a brush or toothpick. Its lacquer based so you can even swipe off the excess with Mr. color thinner and not have to sand it. It does sand well though. For Armor, it works good to simulate rough castings such as on turrets or mufflers. Check out Swannys models website for a great write up.

I mainly use it as a liquid putty to fill small holes and hairline cracks.

I consider it an essential tool for my model building.

One major complaint I have  is you don't have much time to work with it. The liquid in the bottle starts developing a surface film within about a minute of leaving it open and becomes very gummy/stringy.

What ever you use to apply it, small brush, toothpick, also starts to get gummed up.

I've had to add thinner to the bottle because it turned into a "honeylike" paste after sitting on the shelf. Make sure there isn't any dried putty on the inner lid, rim of the bottle, or threads of the cap.

I use the 500 as a filler for small seams, works well on two-piece tank barrels

Since you mention armor it is great for adding casting texture to turrets and hulls.

Great stuff! I use it all the time.