I may be wrong.. but you can clean a rug on the concrete floor of your garage just like you can clean it in a "rug pit".
NO, you’re not wrong and the issue isn’t where you can clean it’s how and with what …..My wash floor like many is basically a concrete floor with side berms to contain the water and a drain, hose bibs , dry tower while it’s purpose built basically a large heated garage. However, it doesn’t sound like you are giving what I’d consider a full wash I’ve highlighted some issues……all may not pertain to you there are some unknowns with what you do.
1. Hang and beat ( like they did in the olden days before the badger ) to dust the rug. OK
2. Vacuum the rug to remove as much dry soil as possible sounds good
3. Hose down the rug with a medium pressure gun making sure all areas of the rug are wet. I’m not sure I know what a medium pressure gun is ..? step 4 should be step 3 …..you getting the surface wet with water that has surface tension and combining it with dirt and then adding shampoo. It sounds like you are not saturating the rug completely front to back ? there is an impedance of soil and water to the shampoo read #4
4. Pour my rug cleaning shampoo mix onto the carpet. Rug shampoos are generally anionic and are formulated for their foam and lubricating properties during mechanical agitation (shampooing) and are difficult to completely remove in a wash because…. Anionic cleaners work best under alkaline conditions the pH. of rug shampoo’s are too low between 7-8 even buffered in most cleaning situations the volume of soil, as well as the natural acidic pH of wool pushes the wash water acidic and under acidic conditions anionic cleaners lose their cleaning properties and favor absorption to wool (protonat) making it impossible to remove soil and the shampoo. You’ll think you’ve gotten the shampoo out the foam stops but it’s really still there. That’s why in-plant rug cleaning solutions for washing rugs are non-ionic built cleaning solutions
5. Take a mechanical or manual agitation to the rug making sure to really work in the shampoo. nap distortion from mechanical agitation , and heavily bleached fringe are the two most common cleaner caused damage I see in rugs. Partway way through a scrubbing protonaton I outlined in #4 begins to happen there’s a loss of lubrication on the textile surface causing friction. I’m not telling you it happens with every rug or every time but it happens
6. Let that sit for 10-15 minutes sounds good we let some rugs soak a full day
7. Take higher pressure water solution and spray the rug down while using a "squegee" method to push the water out of the rug.
Pressure cannot match volume of water when people try it causes damage to the rug most notably nap distortion even pile loss how long do you Squeegee the rug ?
8. Clean the fringe. good, if you wash a rug that's the first thing you should do most cleaner neglect the fringe and hammer it the next day with a bleach
9. Spray down the fringe to rinse the cleaner from it. OK
10. Take pile rake and make sure all the pile is properly lifted to prevent un-natural rug patterns. OK, but it should be laid flat then vacuum to fluff when dry
11. Hang dry or dry on the floor with turbo dryers. OK
12. After rug is dry I will vacuume once again to make sure all the dust is removed prior to wrapping it up for delivery. excelent