Any suggestions for a decent weatherproof wood filler?

Started by fivefingeredstyre, Aug 08, 2016, 11:31 pm

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Title says it all, really.

Its annual maintenance time again. The post caps on my box have started to rot. I'm not surprised as I made them in a hurry out of soft wood, I'm surprised they've lasted this long, considering how thin they are...

My plan is to replace them with resin cast caps; however I want to seal the tops of the corner posts before I fit them. Does anyone have any suggestions on an appropriate wood filler? It needs to be something that doesn't expand if moisture gets in to it.

Any suggestions great fully accepted.


Well not so much wood filler, but 2 renovations back I had around 3 gallons of premixed thinset from a tiling project I'd done years before.  It's kind of like mortar but it's got evil things in it to keep it from setting until it is fully exposed to air--even after sitting in a (sealed) half empty bucket for a couple years.  Since I hadn't beveled my panels, I used it like a glazing putty on all the panels as well as randomly adding some to panels to add concrete texture.  Then I used some to--wait for it--try to waterproof the tops of my corner posts (and the roof steps).  

The instructions say it isn't for outdoor use and there has been some cracking in places where it was laid on very thick, but it might serve your purpose.  

Here's the product:,store:1537580139038499723,lsfqd:0&prds=oid:13757983771857248028&q=SimpleSet+Gray+3-1/2+Gal.+Pre-Mixed+Thin-Set+Mortar,+Grays&hl=en&ei=XGepV8PhD6G4jAOT3IuYBA&lsft=lsf:seller:8740,store:1537580139038499723,prds:oid:13757983771857248028,q:SimpleSet+Gray+3-1/2+Gal.+Pre-Mixed+Thin-Set+Mortar,+Grays,lsfq:AA316erVqZe3dSTOSLWkZ-yS_1DhlLLcO5GOXYhS1pLJ2PrFF1w9SA2Ld3pyc8lIM4kAysaWZg4b-qxkUzYGi71oHg7DvefTPg,hl:en,ei:XGepV8PhD6G4jAOT3IuYBA,gclid:CKHTrb_Ks84CFdKGfgodKxsLTg,gclsrc:aw.ds&lsft=gclid:CKHTrb_Ks84CFdKGfgodKxsLTg  

And I'm having problems with the site but page 3 or so of my build journal should show it being applied.  Looking at the rest of the thread, there was some cracking, so I don't know if you would want to try it after all.
"My dear Litefoot, I've got a lantern and a pair of waders, and possibly the most fearsome piece of hand artillery in all England. What could possibly go wrong?"
-The Doctor.


Hi Steve,
Well I feel a bit odd giving advice to someone whose builds are so much better than mine, but these are my thoughts...
Pretty much every wood filler I have tried has turned out to be rubbish. Even the supposed exterior filler hasn't survived even a year.
Some people use car body filler but it's quite expensive.
I used gutter sealant (Only £1 a tube from the Pound Shop.) The only trouble with this is that it can't be sanded down, which is fine if you want the rough & ready look, but if you want it to look perfect, you need to apply it carefully & smoothly with a putty knife or whatever.
Once it dried, I then covered the whole roof with rubber paint then the exterior emulsion.
The gutter sealant has stood up to the weather very well, much better than filler.

Hope this is of some help.



Thanks Guys

Russ, I know what you mean about 'exterior' filler turning out rubbish, I've had the same problem. They all tend to absorb water and tend to blow within a year or two.

I've tried to use car body filler before, but the problem with that is the wood still absorbs water and as a result flex's. This then makes the body filler crack and flake out. What I've done before is use wood filler and cover it with rubberised paint to prevent the water from getting in. I'm guessing whatever I use I may have to cover with the rubberised paint regardless as this is pretty good at holding back the moisture.

Theta Sigma

"I just put 1.795372 & 2.204628 together." - 4th Doctor


Car body filler is certainly a step in the right direction, but also consider silicon window sealant! That stuff is cheap enough and easy enough to use, and though it comes in tubes with patience you can spread it like a paste into gaps.
On my roof I've also used rubberised paint sealant. It hasn't worked at all, the roof is pulling apart before my eyes... But... I'm more inclined to think that's my fault than the product's because it has such good reviews and I saw how it helped with my own eyes (I reckon the rot set in from underneath, but that's a different story). But expensive but good stuff... If you use it right!
Isn't it how ironic that we have to think of solutions out of the box, in order to build our boxes a lot of the time?


Quote from: fivefingeredstyre on Aug 08, 2016, 11:31 pm

My plan is to replace them with resin cast caps; however I want to seal the tops of the corner posts before I fit them.

My bit of "poor-man's fiberglass"  -- painted canvas glued to the top of my posts and roof is still going strong after a Colorado winter.  We don't get the consistent wet here though... but so far so good. It  keeps boats watertight...  .


In Australia you can buy Builder's Bog it is like car filler but less dense.

The fibre glass industry sells micro cells that are super tiny hollow ceramic spheres.  You can mix them with epoxy resin to any consistency it is by far the best of all the fillers and easily sanded to a fine finish.

To weatherproof protect the wood you can under-paint paint with bitumen paint it is a bit messy and slow to dry but it works well and is relatively cheap.