Judging bias in assessment system with multiple assessors & scoring criteria

I work for a grant giving charity and we use external assessors to help in our selection process.

There are 30 applications for review. We have 6 assessors who will look at no more than 15 app forms each. They have to score each app out of 5 across several criteria.

We total the scores and use these to rank the apps. This ranking influences our decision making on the grant giving.

I presume, each individual application should be viewed by the same number of assessors.

My question is, how do I account for potential differences between the assessors and how strict/lenient they are in their scoring?

It is important the selection process is as fair as possible.

Any help would be greatly appreciated!