Charles...
I asked this question awhile ago when I called DG...I was told the 501 has a heavy cleaner in it for Gel coats with some mild abrasives...
* Cleans and polishes fiberglass surfaces
* Removes oxidation
* Eliminates mildew, and algae
* Retards the growth of additional mildew, and algae
And 105 was in developement for a year and released this year...
The Marine line has more cleaners in the products..
This was the info I was told...
But as always..You Never Know....
AL