How to clean an unsealed wood fence?

A customer wants his fence cleaned. The fence has never been sealed and has pretty heavy mold and mildew on it. What chemicals do I need to pretreat? Is it possible to clean a fence without having to ‘vacuum’ every square inch of the fence? Do I need to post treat with another chemical? I’ve used Olympic Deck Cleaner in the past, but it costs $7.41 per gallon which I don’t feel that is very cost effective. I apply the olympic deck cleaner straight onto the surface with a pump sprayer. I badly wish that I had a 12v setup to apply chems because it takes forever with a pump sprayer as you could imagine. Any guidance/general information on how to clean a fence (unsealed in this situation) would be great. Thanks in advance!

DS a house wash mix on it, let dwell and then rinse. Depending on how deep the root system goes, you may have to do it a few times. I would spray it on one evening and let it sit overnight and come back the next day and rinse, if that is feasible for you. Guess I should have asked first if you have an Injector fro your washer instead of using a pump sprayer.

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Thanks for the quick reply. I should’ve included that info. I have a 4gpm 4000 psi unit. I mainly do houses for which i downstream a house mix. I found that after I applied the ‘deck cleaner’ to the fence, it did bleach or made the fence appear clean where there wasn’t heavy mold, but i found that it left a film on the surface which i had to tediously pressure wash to remove to uncover the clean wood underneath. Is this normal? Do homeowners just want to see that their fence looks new? or is removing this layer of film expected of a professional fence clean? Also, is it normal to have to apply several coats of mix on the heavier mold areas or am I just using a sh***y cleaner for a professional result?

I’m sure there are chemicals out there specifically for cleaning wood. I am hoping someone can point me in the right direction.

sodium percarbonate is also used to clean light mold from non stained wood. Be careful applying too much pressure to the wood or you will take a layer off that fence. Then you will be trying to blend the rest of it in, it’s a pain in the butt. If your unsure, walk away from it.


That looks like PT wood so you’ll need to use pressure, up to 1500 psi to clean after you treat. Easy one. Can use sodium percarbonate or real strong HW mix. Get a 4 gal back pack sprayer if you don’t have a 12v pump. Will make you life a lot easier. Can use it for applying stain too.

With Sodium percarbonate, mixes best if you use warm water and for God’s sake don’t tighten lid on sprayer or whatever you mix it in… It’ll blow up. Leave lid very loose or if has a vent leave that open. Think Hydrogen peroxide bubbles expanding on a much bigger scale.


So pretreat with PressureTek’s F-10, power wash, post-treat/neutralize with F-8, rinse?

I thought I would share a recent experience on this thread as I have not seen to many threads concerning heavy mildew on pressure treated.
I washed a deck for a friend yesterday. The deck/railing is unfinished pressure treated with heavy mildew build up in certain areas. I started by applying about a 2.5 percent HW mix (after wetting the surface with water) but the heavy areas were still green after a significant dwell time. I re-treated with stronger mix until I was applying 5 percent to the bad areas.
They finally came clean (mostly).
I then applied a citric acid solution (1 cup per gallon H2O) with a pump sprayer. I ended up re-applying 2 more times to try & get the color back in the wood. After another good rinse and a re-application of the citric solution, it did get some color back but not as much as I was hoping for.
Also, he had some new boards mixed in with the older deck boards & they ended up with quite a bit of pulp on the surface. I am curious as to why? Was my SH to concentrated? Is new pressure treated lumber prone to “pulping”?
All in all, I was not very happy with the end result. I put in a lot of time & effort for what seemed like minimal return.
I won’t take on a “for-hire” job like this until I get more experience & comfort level with expectations & results.
If anyone does much of this work & wants to offer anything to this thread, I would definitely pay attention. I get asked to bid this kind of work fairly often.

FYI - I found some other threads on this. And am reading up now. Just needed to modify my search.