I have a humidified area for coating and tried different developers (Sodium Citrate and Ammonium Citrate/Sodium Acetate) and clearing baths (EDTA Tetrasodium and Disodium). After trying all different strategies and papers (Platine and Stonehenge), I was still getting yellowed highlights (sometimes subtle but there). So I almost went crazy and finally gave up and have recently been experimenting with Argyrotypes.
I have recently discovered Buxton paper which is really beautiful but expensive (as it is recommended by Mike Ware for Argyrotype) and am now thinking of trying Kallitypes with Buxton?
However, I recently found this in my collected notes on clearing kallitypes:
"The wet-processing procedure requires reagents which perform both the developing and clearing (chiefly the latter). These are solutions of the sodium salts of ethylenediaminetetraacetic acid - otherwise known as EDTA, for short. Two separate clearing baths, each of strength about 5% w/v, are recommended: dissolve 50 g of the solid in each litre of water at room temperature. The first bath is of disodium EDTA, with a pH around 3 to 4, which is optimum for complexing iron(III) and is acid enough to avoid hydrolysis leading to yellow iron stains; the capacity of a one-litre bath will be about 50 10"x8" prints.
Clearing of the residual iron compounds from the paper is improved by immersion next in a bath of Kodak Hypoclearing Agent interposed between the two EDTA baths; the inorganic sulphite in this tends to reduce any residual iron(III) to iron(II) which is then removed in the final tetrasodium EDTA bath; the advantage is that these last two baths have a high pH (around 9) which is optimum for complexation of iron(II) and leaves the paper in a beneficial alkaline condition"
I believe this is from Mike Ware. Though, I just looked and can't find the original.
Once I had discovered EDTA Disodium, I never went back to using Tetrasodium for the reason said of it causing "hydrolisis" but perhaps I will try this process of using both EDTA's ?
Good Luck, It's a beautiful process, I much prefer over Vandyke as it only requires a single coating but the staining of highlights was a constant issue with me.