The ins and outs of plas­ter­ing

handy­mac

Weekend Argus (Saturday Edition) - - GOOD PROJECTS - DON MacAL­IS­TER

THE FIRST coat of your fin­ished prod­uct, plas­ter­ing hides the brick­work and hope­fully gives a smooth sur­face which can then be tiled or painted. Like the mor­tar used in brick­work it is a mix­ture of sand and ce­ment so it is im­por­tant to en­sure that the mix is cor­rect. A mix with too much sand will be too soft and will break up eas­ily. The fin­ished prod­uct must feel hard when you scratch your fin­ger­nails over it.

As the plas­ter will form a “fin­ished” sur­face it is im­por­tant that the plas­terer has the cor­rect tools to do the job; check that his floats of ei­ther tim­ber or steel are smooth and in good con­di­tion. The most im­por­tant tool is his straight edge, check to see that it is in fact “straight” and that the edges are smooth and free of chips.

Ide­ally the fin­ished prod­uct should be 12mm to 15mm thick – plas­ter thicker than this will tend to pull away from the wall. Any area that needs a thicker cov­er­ing, for what­ever rea­son, usu­ally the brick­work be­ing out of level, should be plas­tered in two coats.

Plas­ter that is too thin will tend to craze as it dries out and show the pat­tern of the brick­work be­hind. As an ex­tra pro­tec­tion, wa­ter­proof­ing ad­di­tives can be added to the ex­ter- nal plas­ter mix. Your plas­ter is go­ing to crack; these cracks oc­cur nat­u­rally dur­ing the cur­ing and shrink­ing process which hap­pens af­ter ap­pli­ca­tion. Ex­ter nal plas­ter ap­plied dur­ing strong winds will also show mi­nor cracks, as the strong winds and heat tend to dry the plas­ter out too quickly. These mi­nor cracks are in­evitable and it is un­fair to hold your builder li­able and he can­not be bound to rec­tify them. To min­imise the risks of crack­ing, keep the in­ter­nal walls well ven­ti­lated, and it does not harm to slow down the ex­ter­nal cur­ing process by keep­ing the plas­ter damp for a few days once it has set.

In­sist that your con­trac­tor fin­ishes a com­plete area in one day, as joins in plas­ter will be vis­i­ble. Walls should be plas­tered from corner to corner, and from floor to ceil­ing in one op­er­a­tion, us­ing the same batch of mor­tar. Ob­vi­ously re­veals around win­dow open­ings can be plas­tered later.

Talk­ing of open­ings, check that all frames are pro­tected and that they are washed off af­ter plas­ter­ing is com­plete for the day. As sub­strates such as brick­work and con­crete have dif­fer­ent prop­er­ties, a “v” joint should be cut in the plas­ter where the dif­fer­ent ma­te­ri­als meet; this will en­sure that when move­ment oc­curs the re­sult­ing crack will be in a straight line.

Plas­ter can be fin­ished off in many dif­fer­ent ways, but tech­niques such as stip­pling or Span­ish plas­ter will leave a sur­face, which is more likely to col­lect dust and in my opin­ion should be avoided. For a re­ally smooth in­ter­nal fin­ish, a fin­ish­ing coat of crete­stone can be used; this is re­ally the ul­ti­mate fin­ish and is of­ten left un­painted, with just a clear sealer coat be­ing ap­plied. With the de­vel­op­ments in paint technology an equally smooth fin­ish can be ob­tained by us­ing the ap­pro­pri­ate paint, and the down­side of un­painted crete­stone is that it is very dif­fi­cult to patch with­out the patch be­ing vis­i­ble.

Fi­nally, if you are go­ing to have moulded plas­ter fea­tures on the top of walls or around open­ings, in­sist on see­ing sam­ples first, and agree with the builder on the stan­dard re­quired. There are very few plas­ter­ers left who can re­ally pro­duce good qual­ity mould­ings, so check the qual­ity on a daily ba­sis and see that the plas­terer is us­ing a tem­plate.

Next year – CEIL­INGS

COVER STORY: It is vi­tal that your plas­terer has the cor­rect tools for get­ting the right fin­ish.

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