Custom Sort

Hi, 

I am trying to sort our product list and i have come to a stand still. 

our products look like this. 

there is a name of the PARENT SKU (full product) and under child sku the components. 

i want to be able to filter products by using PARENT SKU Alphabetically for first part of the SKU and custom sort for size aka SIN, SMDBL, DBL,KNG ( i have set up custom sort)

If i sort by RANGE, COLOUR, PARENT SKU (BY GREY FIRST) than SIZE- It goes wrong 

if i sort by PARENT SKU, RANGE (GREY FIRST) , CHILD SKU, COLOUR, THAN SIZE - It doesnt register the size because its picking up on DBL in PARENT SKU BEING ALPHABETICALLY 

Any help would be appreciated 

Thanks 

Lettie