I've used waxoyl before, in aerosol, using the waxoyl sprayer and a compressor and yes, it's messy.
Only the compressor gun worked properly, and pre heat the waxoyl too...
On a reccomendation, I tried out the Dynax range from Bilt Hamber last year.
Admittedly not on my 90, but on my daily drive Lexus RX400h ( it works hard, so I thought I'd treat it when I had it up on blocks to get the wheels refurbished)
Usual prep - remove all the under trays and wheelarch liners, steam clean thoroughly, treat any rust, cover brakes etc, and apply the rustproofing ( I did pre-warm the aerosols - it was a cool day)
Used Dynax S50 aerosol ( comes with a very good spray lance) in the box sections, and Dynax UB on the underbody as a trial. It is available in larger quantities for the pressure sprayer on a compressor, but I thought I'd give the aerosols a go.
I was very impressed with the Dynax Aerosols, they really threw out the product, as good as my compressor setup.
The Dynax S50 went on nicely, covering the box sections interiors well, but didn't come pouring out like I've had waxoyl in the past - the fluid sort of stuck nicely rather than flowing out.... The lance provided a good 360 degree coverage.
The Dynax UB dries off to a harder coating, but never really dries ( if that makes sense)! It stays waxy, but doesn't go hard, but doesn't stay soft and mushy ( like the old underbody sealant).
I had the RX up on the ramps to do the 60k service earlier, and pre-winter inspection, and the Dynax had stood up well, with only a couple of areas that are directly in the "line of fire" of road spray etc showing any deterioration of the coating. The "top up" I applied stuck well to the previously applied Dynax.
Highly reccomended - a real step forward in underbody protection - I'm impressed, and will be using it on my 90 ( if I ever get started on the rebuild).
Being in aerosol format made it a lot easier to use when shuffling around under the car on axle stands - costs a bit more I suppose, but it is a lot easier. Also, it means that it's a two second job to touch it up when you spot something when servicing.
To do the Lexus took three aerosols at about £12 each isn't bad going. I suspect a 90 would use a bit more, as there are more nooks and crannies - maybe up to five?
Good product though - I won't be using waxoyl again...