External Plastering (Rendering on Metal Lath Using St. Astier Products)
The main difficulty in working on lath is ensuring the lath is suitable for the job.
Metal laths come in a wide variety of types and designs, some of which have specific functions and are not suitable for a broad range of rendering applications. It is imperative to check with the manufacturer of the lath what is it designed to cope with.
The lath Manufacturers recommendations for fixing should be sought, however as most fixings are specified to accommodate cement renders or plasters, it may be necessary to decrease the centres of the fixings to 4" (100mm). to reduce to the minimum any movement during application. Laps should be firmly fixed.
Lath can be plastic, metal, galvanised or stainless steel. All can be used with NHL mortars.
Simple expanded mesh lath usually requires greater numbers of fixings to reduce play.
The ribbed laths span more readily between supports and are generally more robust.
Backgrounds: brick, block, concrete or even straw bales. It is also commonly used on framing, either timber or metal studs or to bridge between two completely different backgrounds and supports. If the work spans over two different types of backgrounds, the choice of application and mix will be determined by the weaker support.
Application: ensure that the first coat covers the laths by a minimum of 5/16" - 3/8" from the outside face of the metal (including any ribs or overlaps), particularly where the lath is spanning between framing.
First coat: use NHL 5 and a well graded sharp sand at a ratio of 1:2. The mortar must be allowed to cure and harden sufficiently, at least 1 week in good weather conditions, longer if possible, before a second coat is applied. Internal work should not be force dried under any circumstances.
Further coats can be applied as per the general guidelines for NHL Renders and plasters
(2 or 3 coat work).
Where lath is well fixed to a solid background, use of a spray gun with a NHL 5 1:1.5 can be used for the first and all subsequent coats.
Protection and curing: always follow best working practice, pay particular attention to protection against knocks and bumps on fresh mortars.
Note: if lath has been fixed by another trade, the plasterer should only apply NHL mortars when he is fully satisfied that the lath work is rigid and sound.