Priming badly knotted wood

[Ian]

I've had some woodwork fitted indoors that is badly knotted in places. Will an "ordinary" timber primer be sufficient to cover the knots, or will I need "primer for difficult surfaces"?

[Rod]

Knotting.

[Lobster]

Just to expand on that, assuming the OP's never heard of that, or he wouldn't be asking the question... you need to paint "knotting" fluid over the knots before painting - this effectively seals them and stops them leaking resin etc through the paint. And then a standard primer will be fine. (I doubt that a 'difficult surface' primer on its own would be any more effective than the standard one).

David

[Ian]

From: Lobster Date: Sun, 14 Feb 2010 Time: 09:31:20

David,

Your assumption was correct -- in my very sheltered life I had never heard of knotting fluid. Thank you.

[Stuart Noble]

I wonder whether the whole knotting concept need updating. Although pine resin will bleed through oil based coatings (hence the traditional alcohol soluble shellac knotting) I imagine acrylics are also resistant. Anyone tried water based primer/undercoat over knots?

[JimK]

ISTR nail varnish works too but sounds like you'll maybe need more than a few of 'er indoors' old cast offs...

JimK

[Hugh]

I've had trouble with knots showing through even with water-based and acrylic paints. The knotting fluid (e.g. Ronseal from B&Q) reduces it a lot. I usually paint on two coats as per instructions.

Cheers, Hugh

[Roger Mills]

Yes, and the knots show through!!

[Andy Dingley]

In which case, it's worth noting that knotting is just low quality shellac (and cheaper when bought as shellac).

It's worth having shellac in the workshop - very useful stuff. The typical Screwfix shellac grades still contain wax, which ought to be "dewaxed" before fine finishing work. It's easy to do this: sit it undisturbed on a shelf for a few weeks until the wax settles out, then carefully pour off the good stuff (2/3 - 3/4) into another bottle. The waxy remainder makes excellent sanding sealer (easier to sand, owing to the wax) or indeed knotting.

[Rod]

Indeed a bit of expanding was in order. :-)

Had intended to look it up on the FAQ and post link to that. Other things got in the way so I posted as I did.

Now looking at FAQ can't actually see any reference to knotting. Obviously desirable for it to be added.

[Stuart Noble]

Shellac wax was very pricey last time I looked, maybe because it's about the only hard animal wax

[geoff]

In message , Ian writes

Knot heard of it ?

[The Natural Philosopher]

Use knotting before priming. Mandatory. Then plastic wood for any holes.

[ransley]

Shellac is a product, it is made from an insects secretion not an animal product, it is called Lac. Shellac base primers seal knots well, usualy oil products work but on new bleeding knots Shellac can be better, Here I can get a primer in a spray can for water damaged walls or wood from Zinser and I used to buy it by the can, I think its called Bin. if its a big job you can get shellac and thin it with Alcohol, or even cheap gin, or get a primer with shellac base. No water base product will work like Shellac for all around stain sealing

[gilli]

Hi, I have used knotting only to have the resin leak through it after a few years, having overpainted the knotting with primer, undercoat and gloss. My solution was aluminum-based primer....never had resin get through that. Considering the problems of patching and matching the other coverings, I would never take the risk again..and I would use two coats of the aluml primer too. ! D

[The Natural Philosopher]

none of thse is relevant: You need to use great gobs of knotting.

Ive never had it get through properly applied knotting.

Not that I paint wood much. Or use the sort of wood that needs knotting.

[Rick Hughes]

or varnish/shellac over the knots .. works just the same.

Only any good for Live knots ... any dead knots ... hit them out now & fill, or glue them in, they will fall out otherwise.

[Andy Dingley]

So what are insects these days, broccoli?