I make and use this... works awsome
Here is a recipe for home made "Waxoyl". It's an old fashioned rust treatment prevention / undercoating:
2 1/2 quarts turpentine
12 oz. beeswax / candle wax
1 quart light machine oil
With a cheese shredder, cut the wax into the turpentine, stir until the wax has dissolved, you can use very low heat about 140* degress to melt but be careful) and thin with the machine oil ( i use 0-30 weight)to a brushable / sprayable consistency. Apply liberally. You can use a hand spray bottle to get into closed-off sections if you have a small access hole.
I just dump it in my harbor fart paint spray cup gun and shoot everything under there.
Serious... it will keep the mud down but more important your rig will not rust.
Waxoyl oringinate in Engladnd where it rains 80% of the time.
Please be sensible when you make this stuff; don't go breathing the fumes or applying heat and burning down your house. If you have any doubts about it, err on the side of caution and just buy a commercially available product.