Wood it a tricky topic, it it has a stain or paint you need to strip it, then to balance the ph and get rid of those black stains or tint you can use a wood brightener, it also restores the ph. Just be careful with the power-washer not to get to close
and then at that point you can apply your stain, clear stains to dot help to prevent uv rays from harming the wood
if you are hiring someone they should know what there doing i always use laundry detergent with bleach in my soap dispenser apply with low preasure starting from the top to bottom then switch to hi pressure starting from the bottom up if you need to restain use a quality waterborne stain like defy, cabots or b/moore they already have uv protection in them they might cost more but the results will make you happy for years to come
I do not like pressure washing wood. What you are doing is driving water into the wood wherever it is open and no matter how careful you are, some will make it's way past the siding and that is never a good idea. What it really is is a cheap easy way to get out of doing the work properly by hand.
With 8 years on the paint now I'd be repainting at this point, more to protect the original paint and it's bond than anything else because that is what every coat of paint is going to count on hereafter. Take care of that and you will avoid most future paint woes.
I always power washed homes that were to be painted, but also those that were to be merely cleaned. I used household bleach into which I shot as much Ivory Dishwashing Liquid as would fit in the top of the plastic jug. Ivory is a mild natural soap that will not hurt any vegetation. In Fact, Ivory is often used to get rid of aphids on house plants. But the main purpose of the Ivory was to foam up and indicate to me that the bleach as well had hit those areas.
Mastercarpentry is correct, just like washing interior walls, you always start from the bottom up when aplying the cleaning solution. This is to avoid dirty water running down a dry wall and making streaks which might not come out.
When using strong bleach solutions, you want to stand back and thoroughly wet down all the vegetation which might be hit with the bleach. Then you wet the entire section of the house down with the solution and keep it wet for about 5 minutes. DO NOT let it dry on the wall. Next give a vigourous, systematic rinse of the wall, but not so hard that you damage the paint. Finally, rinse down the vegetation.
On the typical house, I would go through 3 to 4 gallons of bleach/soap solution. I never had a complaint of damage to the house or vegetation. The bleach only does damage to the vegetation if it is left on the leaves. What goes into the ground is so diluted it does not seem to matter.
I actually used a high pressure soap injection tip which replaced the normal tip. This tip could shoot a stream up 15 feet or better. This could hit the eaves of a two story house from the ground.It also meant that I did not have to wait for 50 feet of hose to clear out when I changed from solution to clean rinse water. For those high areas, a zero degree tip could be safely used. Lower down, I would change to a normal spread tip.
I personally don't believe significant almounts of water get into the siding. Where possible, I tried to shoot at a slightly downward angle, rather up the clapboard laps. The Benjamin Moore manual highly suggests power washing of all surfaces to be painted. Further, a properly constructed house would have back-primed wooden siding and some form of rain screen behind it.
The average power washer is putting out about 3 gallons per minute, at about 3000 psi, give or take. Unless you are bareing down and holding the tip in one place, a very minimal amount of water is put on any given area.
In my opinion, homes that exhibit peeling, are not doing so because of powerwashing, but because of some intrinsic construction failure, usually from the inside out.
High Pressure Injection:
the only reason a person don't like to p/wash is because they don't know how to do it right i have been doing this for over 30yrs witout 1 problem when i started to paint we didn't have a p/washer this has been a labor saving device to us painters it's a wife tale about raising the grain story same tale about using water based stains for woodwork been using it since '93 i would never go back to oil passe
Ordjen, if you're watching this forum, your description and recommendations were extremely useful and well explained. Thank you!
Your welcome. I, like Masterpainter, have been powerwashing for years. In my opinion, powerwashing has probably been the best innovation in housepainting in decades! That, and the advent of modern acrylic housepaints, have made for easier application and longer duration of the paint job.
i powerwashed a garage once and blew big chunks off!