Too thin... in the end I did it without washers. I just was very careful with the screws, only putting them in far enough to hold the rack, but not to contort it. It's up now and the only issue is a slight bit of plaster that came away from one of the edges.. I'll fix that once I'm done with everything else.
It's a bit reassuring to have found out the reason wasn't that I'm a complete and utter idiot, but that the rack was slightly wrong.
It's a bit reassuring to have found out the reason wasn't that I'm a complete and utter idiot, but that the rack was slightly wrong.