Quoted from BeachPickle:I’ve been planning to do this and have pondered the same thing..
More often than not you will see people put PETG washers below the plastics, but I will be putting them above the plastics. My rationale being - fragile artwork is printed on the bottom of the plastics, whereas the top side can be polished if it becomes a little worn from rubbing, as opposed to having permanently scratched artwork. As long as the washer extends past the edge of the plastic, it should reduce the potential for busted plastics.
As far as I can tell the only reason you’d put them underneath is to make the washers less noticeable visually?
A better approach might be to get plastic protectors which go underneath the entire plastic, removing any possibility of something rubbing against the Art. These serve the same purpose - protecting the plastic (usually sling plastics) from breaking
