yes, what has happened to arches papers? i thought my occasional bad experiences with their 600gsm sheets (specifically the double elephant or 29" x 41" sheets) were just luck of the half irish. what kind of bad experiences? how about a tuft of what appeared to be lint mixed with human hair stuck in the middle of the sheet, buried in the pulp, that had to be carefully trimmed down to the surface? or, in another sheet, a patch of denser pulp that repelled paint even when the patch was scraped slightly and scrubbed with a brush as paint was applied.
then a painter i admire wrote me about problems *she* was having with arches sheets, problems she took to the manufacturer and to which Arches responded in a generous manner. but one off gestures to repair the relationship with the artist do not get at the underlying problems with the paper.

case in point: the 29" x 41" figure nude of sienna, which i had to abandon because of inexplicable blotching across her ... well, across the part of the image that i could not disguise with texture, pattern, dark values or strong color contrasts.
i know how to store and handle papers; i wash my hands before handling papers, and i do not bruise or abrade papers accidentally or on purpose. these blotches get darker grossly darker when the paper is wet, implying the flaw is in the pulp.
the companion painting disclosed a different flaw -- a small cluster or spray of white dots, each about 1mm in diameter, covering an irregular, elongated area about 6 cm/sq. these dots, whatever they are, repel paint and cannot be worn down or lifted by judicious scraping with an xacto knife. they appear in a mid valued area where i can probably disguise them with texture.
i am a habitual user of Arches watercolor blocks, both the 300gsm and 600gsm sheets, and in general the 300gsm (140 lb.) blocks have a rock solid consistency. i recently purchased some 20 year old 300gsm blocks from an artist who wanted to offload her inventory, and the quality of the 300gsm sheets 20 years ago and today is indistinguishable.
in contrast, the 600gsm block sheets seem a little erratic to me; the surface in particular seems less finished and more unpredictable. so i have a conjecture: all the 600gsm weight papers are made in a separate line or at a separate manufactory, under separate or subsidiary management from the higher volume plants. if so, someone from Arches corporate (well, Arjo Wiggins or whoever owns them now) should drive out to that plant and do a quality and process review. something unhappy is happening to arches papers.