I usually mix 2 parts waxoil 1 part thinners (or old engine oil if it is going in the chassis). Make sure that the waxoil is pre-heated (leave the tin in a bucket of hot water).
I mix it with a 20 % diesel, Anything oil based will dilute it,
Never tried the thinners though
I like the diesel cos it helps the waxoyl to creep a wee bit more in to the joint, I have used white spirit and petrol in the past but i find diesel works best. I mix it up and then use a parifin gun that way you can adjust the flow
Brushing is fine - but remember that most chassis rot from the inside out. You need some way to get it inside the chassis or you're just wasting your time.
Its worth buying the pump and spray attachment, it wont break the bank and you insert the tube into the chassis and pump up the pressure and fire away. You must heat up waxoyl to make it runny, place the tin in a pail of hot water. Remove the lid to release pressure, pour into the spray cylinder and off you go. Place drip sheets on the ground because it will get everywhere.
If you haven't already bought the Waxoil - have a look at Dinitrol chassis wax. They will supply it in spray cans with an extension tube at no extra cost.