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Subject: Paint blistering

Paint blistering

From: Rick Kerschner <rkerschner<-at->
Date: Wednesday, October 28, 2009
>We used an acrylic paint supplied by a well-known manufacturer with
>a primer made and recommended by the same manufacture. Half of one
>side of the house on the South side had new clapboards. These
>clapboards painted at the same time did not blister. The clapboards
>on the rest of the house are in excellent condition with some firmly
>attached lead based paint on them but the paint put on five years
>ago is coming off in pieces as large as your hand.
>
>If the blisters are caused by the expansion of the paint while
>elastic, how is this prevented?

There is a good possibility that the new clapboards were back-primed
while the older ones were not. Back priming is very important to
prevent the clapboards from absorbing moisture from the back and
expanding and then contracting when the sun hits them and they dry
out. The movement of the boards breaks the bonds between the paint
and the wood, bonds that are probably already relatively weak on the
old boards unless they were completely sanded to "new" wood before
painting. Recent research indicates that wood deck sealers that
contain some wax work even better for back-priming than exterior
paints.

Rainwalls to allow air to flow behind the clapboard will also help
the clapboards retain paint. A new easy-to-use rainwall material is
available at

    <URL:http://www.benjaminobdyke.com/>

I have had a similar experience on my 100 year old home. Paint on
new boards on the south side has lasted 10 years and that is the
side that is subjected to the most brutal weathering from the sun. I
recently added a new top coat of paint because of chalking, but no
scraping of loose paint was required because there was no loose
paint. Old clapboards on the other sides of the house require
scraping and repainting every 5-7 years. I am gradually replacing
the old clapboard siding, working my way around the house a year per
side in the old New England fashion. This year it was the west side
and the east side is next. The north side is in the best condition,
probably because it never sees the heating and drying cycles from
direct sun.

Richard L. Kerschner
Director of Preservation and Conservation
Shelburne Museum
PO Box 10, Route 7
Shelburne VT 05482
802-985-3348 x3361


                                  ***
                  Conservation DistList Instance 23:15
                 Distributed: Friday, October 30, 2009
                       Message Id: cdl-23-15-004
                                  ***
Received on Wednesday, 28 October, 2009

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