Cleaning Oak Frames

Cleaning Green Oak

When oak is delivered from a sawmill, it invariably has bluey black stains on it. These are produced by the iron in the saw blades reacting with the natural tannin in the oak. In the workshop further marks are put on the oak by the carpenters, snapping chalk lines and facing the timber. The finished beams are then transported to site which often is muddy and wet causing them to get dirty. If it rains after the frame has been erected, brown water stains are produced on it by water leaching out the tannin in the oak. Added to all of this, dirty marks left by builders fixing the external envelope and it’s no wonder the frame needs cleaning.

Sandblasting

The most common way to clean the oak frame is by sandblasting. Doing it by other methods, such as manual sanding or planing each face takes an inordinately long time and can prove to be costly. Sandblasting isn’t cheap though, so the cost should be factored in to the build at the planning stage. Quotes can usually be supplied from by specialist sandblasting companies, working off the framing drawings. Sandblasting works by shooting grit, feed from a hopper, at an extremely high pressure (100psi) through a nozzle. When the grit hits the surface of the oak, it removes a thin layer of wood leaving clean bare timber behind. This can appear quite white and a bit patchy just after the sandblasting has been completed, but within a month the surface of the oak re-oxidises and begins to turn an even golden colour. The blasting needs to be done with quite a light touch (if such a thing is possible) so the grain of the oak is only slightly raised but not blasted so much that it resembles driftwood. This is achieved by jetting the grit in a sweeping motion along the direction of the grain, never letting the nozzle rest in one place.

The type of grit used in blasting green oak is extremely important and disastrous results can occur if the wrong product is used. As previously mentioned iron will react with tannin to create a black stain, and unfortunately most types of sandblasting grit contain small iron particles. If an oak frame is blasted with a grit containing iron particles, initially everything will appear to be fine. After a couple of days though, the whole frame will start to turn black, as iron particles blasted into the frame react with the tannin. Once this has happened it can be difficult to reverse, as some of the particles can be deeply embedded in the oak and re-blasting will not always remove them. Luckily there is a non-ferrous grit available called ‘SC,’ which is clean and safe to use. If there is any doubt about the grit, a sample should be left on some green oak for a week to check for any reaction. After this time, if there are any signs of discolouring it should not be used. Given the risks associated with using the wrong type of grit, only sandblasters with a proven record and thorough understanding of blasting oak frames should be used.

The timing of the sandblasting is quite critical within the overall building process. It is after all, the finishing treatment for the frame after which no further beautification is needed. The problem is though, that grit blasted out at 600 miles/hour can easily cause collateral damage to materials placed next to the oak. This is especially a problem with soft materials such as plasterboard, which may have been placed on top of the rafters prior to roofing. Any over-blast when doing the rafters could easily rip a hole through it. On the other hand though, if the frame is left unprotected after it’s blasted it could be covered in water stains from rain in no time (this is Britain!) One solution is to cover the frame with a heavy gauge polythene, which can easily be acquired from a builders merchant, soon after it is erected. Once the frame has had a chance to dry out for a week or two, it can be blasted without fear of ruining any other materials, and if a hole is put in the polythene, it can be repaired simply with some gaffer tape. The polythene can then be removed section by section as the external envelope is built. If the rafters are covered with a timber sarking it is possible to blast the frame after the roof is watertight. There will inevitably be damage to the sarking, if it has been pre-painted, but it is possible to touch this up after the blasting has finished. Sandblasting by its nature, is a messy, noisy business and requires a fair amount of clearing up afterwards. With all this in mind, contact should be made with a suitable sandblaster well in advance to coordinate the best time to blast the frame.

Steam

Which ever method is used to protect the frame after sandblasting, it is almost impossible to not to get a few water stains in the oak before the envelope is completely watertight. These can be washed out with a scrubbing brush and some warm, soapy water but the whole beam will need to be done and not just the stain otherwise a ‘tide mark’ will be left. A better way to clean them off is by using a steam cleaner. DIY stores often sell wallpaper strippers which come with various attachments, one of which is used for cleaning upholstery. This can be wrapped in a cloth and used to clean off the water stain by rubbing it over the affected area. Because the steam evaporates immediately, no subsequent stain or tide mark is left. Obviously steam is very hot, so protective clothing should be worn.