Mechanical sanding tools have given the industry a great advantage in time saving on the job particularly for sanding large areas. As is the case with any innovation, the benefits have some tradeoffs.
The AWCI has done some research, regarding the cuffed or rough surface left when mechanical sanding machines are run over topped screws and joints and asked manufacturers, users and technical advisors for their correct sanding techniques and instructions.
Some of the issues are:
Topped screws are usually 100mm wide and the sanding surface of mechanical tools is 225mm in diameter. Topped screws for the exercise are regarded as having 2 coats of base coat and 1 of topping.
Some sanding gangs not only sand the screw area but go along the full length of the plasterboard, along the line of screws as well as going at right angles to that line at the end of the walls.
Some sanding gangs seem to be unaware of the requirements of the Australian Standard 2589:2007 which says the finish is no acceptable if the edges of the joint cement/compound have been over sanded, resulting in scuffing of the paper surface.
Observations for topped screws and finished joints
This issue is not unique to Australia but world wide. All local plasterboard manufacturers virtually have the same face paper on their plasterboard (called paper liner board) so the issues of scuffing are not related to differences in paper stock.
Powered dustless sanding systems were first introduced to the plasterboard trade over 10 years ago, revolutionising the sanding of plasterboard and saving hours of unhealthy labour otherwise due to the use of traditional manual methods.
Plastering contractors need to supervise the sanding gangs and bring to their attention the requirements of the Australian Standard AS/NZS 2589.
It is absolutely critical that plastering contractors set out their requirements such as not to oversand or create scuffed surfaces and be present when the gang first starts and get them to do small test areas so they can be suitable instructed. Some plasterers manually sand a test area to show the mechanical sanding teams what they require. This is of great assistance.
There are many different types of compounds, most of which are quite easy to sand nowadays, but require different techniques and abrasive grits, usually from 220 to 260. It is important to use as little pressure as needed to product a fine sanded finish and not to stay too long in the same spot. Optimum performance of mechanical sanders also involves the discarding of worn pads before they become ineffective. Over zealous and inexperienced sanders can destroy the surface in seconds. Be mindful only to sand areas that contain compound.
Manual sanding of topped screws
Whilst mechanical sanding machines play an important role in our industry, let’s not overlook the great results you can still achieve by manually sanding the screw area.
The traditional technique is to sand the topped screws with a foam sanding block – preferably the large size.
Foam sanding block should also be used to finish fine details of all surfaces. Many finishers do not pay attention to fine finishing details, thus leaving the painter to tidy up rough work.
Read below for solutions to commonly occuring problems with mechanical sanding systems:
Problem: Sanding task often given to inexperienced members of the team
Solution
All users should be fully trained in the proper use and technique of power sanding systems, including the correct machine speed, correct abrasive grit and suitable machine for the application. This should form a larger part of all plastering courses.
Problem: Under sanding of the finished joints
Solution
Machine speed is either too slow or the abrasive is either of inferior quality or too fine a grit for the compound used. Try either a faster machine speed &/or a coarser grit for the compound used. All jobs should be inspected by the site foreperson prior to signing off.
Problem: Over sanding of the finished joints
Solution
Machine speed is either too fast or the abrasive is either of inferior quality or too coarse a grit for the compound used. Try either a slower machine speed &/or a finer grit until the correct finish is achieved. All jobs should be inspected by the site foreperson prior to signing off.
Problem: Gouging of the surface
Solution
Often caused by the use of inferior designed machines or careless technique. This problem generally occurs when the sanding pad is placed against the ceiling surface with the head of the machine at an incorrect angle. There are sanders available with flexible preset head angle for ceilings, ensuring the head is at the correct angle when the pad is placed on the surface.
Problem: Scratching of the surface and swirl marks
Solution
It is mostly the use of inferior pads that cause many of the problems with plasterboard joint sanding, including the clogging of the dust extraction holes. It is recommended if you are encourtering these issues that you consult with your supplier on site, so they can observe firsthand what you are working with and give you assistance in creating a good finish.
Problem: Uneven sanding, or skip marks on the surface
Solution
This is mainly due to the Velcro backing pad on the sanding machine not being centred on the sanding plate or damage to the foam backing. This causes vibration, resulting in uneven sanding and “skipping” of the sanding pad on the surface, cuasing marks accross the finish.
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Article courtesy of AWCI On The Surface Magazine, 12th Edition – Summer 2011
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