Sand­blast­ing dis­coloured cedar a no-no

Winnipeg Free Press - Section G - - HOMES - ARI MARANTZ

QUES­TION: We have ex­te­rior wavyedge cedar on our home and garage. Over the years it has started to darken or in some spots turn black. We are won­der­ing what is the best way to clean it as we would like to re-stain with a clear color this year. I was think­ing of hir­ing some­one to sand­blast the build­ings. Is this a good idea?

Jen­nifer Stock­mann

Most wavy-edge cedar sid­ing pro­duced in the past was high­qual­ity lap sid­ing, of­ten made from clear or al­most knot-free cedar with a rough sur­face. It is in­deed worth try­ing to save this ma­te­rial and re­new the fin­ish, but you must take care not to go over­board or you may dam­age the soft wood. My an­swer to your ques­tions will pre­vent this from hap­pen­ing, if you fol­low the ad­vice pro­vided.

Sand­blast­ing is a method of clean­ing or re­new­ing the sur­face of older ma­te­rial by bom­bard­ing it with fine sand un­der high pres­sure. This is of­ten used on older brick, con­crete, or ma­sonry sur­faces that can with­stand the highly cor­ro­sive na­ture of this process. It can also be used on older wood, but is usu­ally re­served for hard­woods or old fir beams and col­umns that have sub­stan­tial thick­ness and strength. To my knowl­edge, it is not used on soft wood sid­ing, like cedar or pine, be­cause that ma­te­rial will not be able to with­stand the force of sand­blast­ing with­out dam­age. For­tu­nately, there is an­other sim­i­lar process that may be ideal for your pur­pose.

In­stead of blast­ing the de­te­ri­o­rated sur­face of your old sid­ing with sand, you should be able to use a pres­sure washer that uses wa­ter. These de­vices are read­ily avail­able for ei­ther pur­chase or rent at a mod­er­ate price. Since they only use wa­ter to re­move sur­face fin­ishes and dirt from the sid­ing, other than cov­er­ing bushes and veg­e­ta­tion to pre­vent dam­age, there are no neg­a­tive ef­fects on lawns or land­scap­ing. Pres­sure wash­ers are now in wide use by painters and oth­ers for prepa­ra­tion of a va­ri­ety of sur­faces prior to paint­ing or refin­ish­ing, due to their ease of use. They are nor­mally avail­able in ei­ther elec­tric or gaspow­ered mod­els and re­quire a lawn faucet or other wa­ter source, which is usu­ally present on the out­side of most homes.

While the wa­ter will do lit­tle to dam­age any­thing out­side the home, care must be taken not to spray too di­rectly to the sid­ing, to avoid dam­age and mois­ture in­tru­sion though the walls of the home. Win­dows, doors, and seams in the sid­ing may be ar­eas where wa­ter can eas­ily pen­e­trate the build­ing en­ve­lope if ap­plied di­rectly un­der high pres­sure. Us­ing the pres­sure washer at an an­gle, at a rea­son­able dis­tance from the walls, should pre­vent these is­sues from hap­pen­ing. Re­cently, I used a pres­sure washer on my cedar deck at the cot­tage to re­move old fin­ish and dirt. I was sur­prised at the rel­a­tive ease in re­mov­ing ac­tual fi­bres from the boards, if care was not taken to pro­ceed slowly and at a safe dis­tance from the sur­face. The ben­e­fit of pres­sure-wash­ing your older rough sid­ing is that the sur­face should be al­most per­fectly pre­pared to re­ceive the new fin­ish, once it has dried. You may have to do some mi­nor sand­ing of rough ar­eas or ones with ex­ces­sively raised grains, but oth­er­wise min­i­mal prepa­ra­tions should be re­quired.

While clean­ing the sur­face in the man­ner sug­gested may be the most cost-ef­fec­tive and be­nign method, it is not the only one avail­able. Use of chem­i­cal paint strip­pers may also be an ef­fec­tive way of re­mov­ing the old fin­ish. There are ex­te­rior strip­pers that are safe for plants and grass, but the cost may be pro­hib­i­tive. Also, ex­ten­sive scrap­ing and sand­ing may still be re­quired af­ter ap­ply­ing the paint strip­per to re­move heavy fin­ish ma­te­rial. Be­cause this method uses cor­ro­sive chem­i­cals, there can be ar­eas of the sid­ing that may re­quire mul­ti­ple coat­ings and still may ap­pear un­evenly cleaned when com­plete.

Once the sid­ing has the re­main­der of the old fin­ish and stains re­moved, there are sev­eral op­tions for refin­ish­ing. The first thing to know is that the grey and then black dis­coloura­tion of the wood is not nor­mally due to rot or de­te­ri­o­ra­tion, but from the ef­fects of the sun. Ul­tra­vi­o­let (UV) light will turn the wood a nice light grey, ini­tially, but this can progress to dark grey or black if left to progress. If the dis­coloura­tion is so ad­vanced that the wood does not look rel­a­tively uni­form in colour af­ter pres­sure-wash­ing, you may have to pre­treat the sid­ing prior to fin­ish­ing. There are paintable ma­te­ri­als avail­able that re­place the nat­u­ral chem­i­cals in the cedar, elim­i­nat­ing the grey and bring­ing back the nat­u­ral tones. These are only ef­fec­tive on un­treated wood or once all the old fin­ish is re­moved, which should be pos­si­ble af­ter pres­sure-wash­ing. These coat­ings are of­ten costly, so they should be only used if the sid­ing is quite badly dis­coloured.

The fi­nal thing to ad­dress is the ma­te­rial se­lected for refin­ish­ing. While you may de­sire a nat­u­ral or “clear” colour, as you sug­gest, that may not be pos­si­ble on older wood. Be­cause fur­ther dis­coloura­tion will be caused from UV ex­po­sure, se­lect­ing a fin­ish with high UV re­sis­tance is crit­i­cal. Most of the prod­ucts with this built in have a sub­stan­tial tint, which helps in keep­ing out the harm­ful rays of the sun. There are many prod­ucts de­signed specif­i­cally for cedar that will give you the most nat­u­ral look, but don’t cheap out. This is one area where you nor­mally get what you pay for and the pricier stains or fin­ishes will last longer and give you a more nat­u­ral­look­ing re­sult.

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