Help with Potassium Dichromate on Sapele

[scott vroom]

I'd like to get this thread back on topic. I've built a large desktop slab from sapele and am experimenting with potassium dichromate as a finish option. The sample board I'm holding has been treated with 1/2 tsp PD in 16 oz water to darken/oxidize the wood, then top coated with clear poly. This is more or less the look I'm shooting for. Problem: Another pic shows where I tested the PD on 6" of the slab end....no top coat applied. The problem is in what appear to be water spots (in white circled area) that appeared when I applied the PD. The closeup pics shows the affected area more clearly. I applied the PD with a paper towel evenly and quickly wiped off the excess. I cannot figure out why these spots appeared. The wood has been sanded and there was no evidence of pre-existing water spots. My concern is that I apply the PD to the entire 31 sq ft slab and have these spots appear elsewhere. PD is nasty stuff and I don't want to have to sand it out if. Anyone have any thoughts on why I'm getting these spots?

[Mark Bolton]

Do you recall how much you trimmed from the ends? Have you done a couple test boards to see if these show up further in from the end grain?

[scott vroom]

Mark, I trimmed about 6" off the end of the board in question. I sampled the PD on about a half dozen of those 6" cutoffs and saw no evidence of the water mark look. Sounds like you're thinking there might have ebbn some sort of end grain contamination that caused the effect. What's weird is that when i wiped on/wiped off the PD, that one area remained wet even after my heat gun had dried off the other area. Anyway, I'm posting a pic that shows that area with 2 coats of clear poly brushed on.....the "water marks" are visible but their edges are more blended and they don't look so bad/pronounced. The overall finish look is pretty much what I'm looking for and I'm close to moving ahead with the PD on the entire slab. To be safe, I'll flip the slab over and apply to the bottom to see if there are any more water marking issues. Thanks for your input.