I bought one of those army goretex smocks after a reccpomendation on here and have pointed others at the ebay spource.
I have used mine for about 6 months, it has been mostly waterproof, but it is really dirty now.
How do I wash this without ruining it?

Is it just dirty, or is it wetting out?

I wear mine in the shower when I get back from a ride to get the lumps of mud off and then periodically use tech wash and this seems to keep my jackets water proof for a long time. When they do expire it is usually on the shoulders where packs rub.

I use Nik Wax tech wash on my Goretex/waterproofs. Make sure there’s no wash powder or conditioner residue in the machine by doing an empty wash first then wash using tech wash on 30 degrees. I usually tumble dry on low afterwards. I’ve not found a need with Goretex to do anything else. With conventional coated waterproofs I use the TX direct wash in re-proofer afterwards.
Hope this helps.

Its covered on mud right now, I haven’t been in actual rain for a couple of weeks.

Get in the shower wearing it later, then leave it dripping when you get out. You can transfer it to the airing cupboard when the drips have stopped. Wearing it I find is better than holding it under a hose pipe or the shower. See how it goes after that.

Would pure soap wash work?

I would think so, it’s the detergents that cause problems. I think the flakes are best.

I have pure soap liquid? Will it be OK?

I would think so, if its pure soap (carrier excepted). I have used hand wash for wool and delicates on softshells with no apparent adverse affect. I would still try cleaning the mud off in warm water first.

Yep liquid soap for washing (same as tech wash) rinse then soak in TX, tumble dry or waft a hairdryer over the outside to get some heat to ‘activate’ the TX but be mindful of the seam tapes getting unglued with the heat if you tumble the shit out of it

I use a large plastic builder handled tub to hand wash my outdoor gear in – guarantee to be free from washing powders and you can really soak in the TX

If it is Goretex I don’t believe that you need TX. The material itself provides the breathable magic!

If you have a tumble dryer then once it’s clean it will benefit from a low heat drying session – it helps to reactivate the proofing.