Hey all, this is my first post in....well...YEARS!
This is what we did with one that developed some sort of weird allergic reaction in our show barn. The owner wasn't available to approve testing, and the time-frame was too short before Regionals.
Ingredients you will need: One gallon of white vinegar, one gallon cider vinegar, 1-2 boxes CORN STARCH, 2-3 boxes BAKING SODA. Generics are fine.
We put CORN STARCH in a shaker bottle, dusted her body down well, then rubbed it in with a rubber curry. Rinsed well with hose.
We made a thick paste with BAKING SODA and water. This was rubbed into all dirty spots, white areas and mane and tail. In a spray bottle (quart size) mix 1/2 water, 1/2 WHITE vinegar. Spray this everywhere you applied the baking soda mix, and rub each area thoroughly as it foams. Keep spraying each area and rubbing until it stops bubbling. (Or until your arms give out, whatever happens first.
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Rinse well with plain water.
Last, fill a 5 gallon bucket with water and add 5 cups CIDER vinegar. Sponge over the entire horse until the water starts to bead off. It will look like a windshield that you've applied Rain Guard to. One 5 gallon bucket should be enough for a mini, it took 2 full ones for a 14.3 hand Arabian with full mane and tail.
The cider rinse neutralizes the other things, pulls the natural oils back into the coat, gives a soft touch to manes and tails, and is also a bit of an insect repellant.
We did this to a mare the day before hauling to Regionals (They were within an hour of us that year) and she took Regional Champion 3 year old Arabian filly and Reserve Regional Champion mare. The trainer I apprenticed with didn't believe in "goop", he believed in elbow grease. The only clipping she received was on her 4 stockings, a modest bridle path, and the long hairs of her muzzle and jaw line. Just before we went into the warm up ring, we rubbed corn starch into her stockings and blaze, then brushed out the extra. She was one slick looking, chromed-out chestnut filly when she went strutting into that ring!!
I know this all is a bit more complicated than lather/rinse/repeat, but the results are well worth the effort.
Good luck!! Please keep us posted on how things go at the show.
Edited to add: now I feel all pumped up to go out and give my guy a spa day!