It's all about bacteria being trapped in the scratch, then causing an infection. This will not be a danger in a mash tun. Mashing temperatures are too high for most bacteria and the subsequent boiling will do 'em in for sure.
Thanks guys. Bought the tun and never used it and the diy strainer is sharp and caused scratches. I was all in to get a grain father until I was told it only made 17 litres of wort. Think I will get an urn from nisbets and make 30 litres.