Micro- and hydro-sandblasting for wood and wood coatings
L Podgorski, V Georges, P Svane
Exterior wood coatings are getting increasingly durable and often more film forming. As a result they become difficult to renovate. Conventional renovation techniques (chemical or thermal stripping, sanding) are demanding techniques which take a long time to implement. Therefore renovation techniques are requested which are easier to use and which ensure that the wood surface is suitable for a re-coating.
Abrasive sandblasting is commonly used to restore buildings. However traditional sandblasting is too aggressive for wood especially because of the pressure generally used in such a process. There are two renovation techniques which are of interest: hydro-sandblasting and micro-sandblasting, these are both renovation techniques used in other industrial sectors and operating at lower pressures. In the first method a mixture of water, abrasive particles and air are propelled towards the surface. The second one is similar, but operates without the addition of water. This paper presents different investigations carried out with these two surface cleaning techniques on wood and wood coatings. The influence of the abrasive media was studied and showed that plastic or vegetable media were not effective whereas crushed glass grit gave good results.
The investigations have shown that these methods both are very efficient to remove old coating material from wood. When aged uncoated wood samples are considered, the techniques allow the samples to get back free of greying. Three coating systems have been used to study the re-paintability of blast cleaned wood. Natural and artificial weathering results highlight the importance to use coatings systems with a high solids content when sandblasted wood has to be re-painted in order to cover any surface irregularities created by the sandblasting process. Scanning Electron Microscopy has revealed that sandblasting particularly affects the structure of the earlywood and leads to cells compression. The abrasive techniques leave a topography reflecting the variations in wood hardness. With Electron Diffraction X-ray analysis some glass particles have been detected in the wood structure. Comparing their initial sizes with those measured inside the wood shows that the impact with wood during the cleaning break the abrasive particles. Their presence in the wood structure do not seem to affect the performance of the stain used for renovation.