Title Key points on WB
Name Admin Date 2020-11-03

Key points to be checked / major paint defects under the current water borne paint system:

With respect to the paint defects under water borne paint system whose starting year in South China in 2016, whole China in 2017, still there have been considerable problems happened at the time of production in factories and rechecking it at depots even in warranty periods. the main reason has been still in man-made mainly without following painting technical procedures correctly completely. for which we want to analyze it deeply again technically logically to improve it ASAP.

a) Production layout updated in the WB painting line:

As shown on the diagram here; especially for the Zinc/Mid coating dry booths ( the very essential ); where factory equips to put 4 units of 40'HC dry in the dry oven with optimum space, temp / moisture and air blow ducts, etc because those dry / time ( mainly for interiors ( ceiling / front panels ) are very important under WB paint.

in the following diagram: pre-coating means zinc stripe coating

 Note: as asterisked () above ; those 3 stages are auto-paint spray stages for the zinc, mid & top coatings;

       its main spray mechanism is as sketched below in detail:


    b) paint drying time per each paint supplier into dry booths in comparison with total output per day vs. tack  time a unit:


b-1) paint dry time per factory into the dry booths as per each customer vs.total daily output of each customer for 10 hrs. working a day:



  Note provided No PELT applies for the paint DFT checks, especially the ZINC / MID coating, nobody knows those actual paint DFT when painting has been completed. As we know the ZINC primer, ( in general, 30 microns in the spec. ) which is very important for anti-corrosion on the steel substrate, is the main reason why all the painting suppliers are requiring its drying time into dry oven at the very least  12 minutes a unit under their own analysis of the water borne ZINC. Besides. its longer time requiring by Kansai than others as noted above in item ( #b )


c)  Likelihood clues of paint defects taken place technically / logically:

  On the basis of actual condition / data noted items aforementioned (a,b,b-1); we can draw a clear result arithmetically pursuant to paint suppliers' technical radical data recommended objectively ( rationally, there're no other higher technical experts being familiar with their own paint technical back-data and paint itself nature taken from the paint material analysis than the paint suppliers, thus container manufacturer / buyer have to respect them and their technical recommendations as always on the spot ),requiring the standard data to let manufacturer follow as much as they can to keep an optimum paint quality level, especially at the ZINC / MID coating booths without creating of paint defect and with a longevity of container life eventually.

More in the concrete:

When: at the time of production of the container in factories; total output a day, in- between 220 units and 260 units production


What: the nos. of output different by 40 units a day can be resulted in and causes possible serious paint defects absolutely:

     provided 4 units of 40'HC day into the dry booth being put as shown on the diagram (a) above; we can calculate staying

     time of the container into the booth a unit is:

     At 'A' group factory ; the 220 units output category : 600 min a day ( 10 hrs working a day ) ÷ 220 = 2.73 min a unit(tack

     time a unit during producing ) x 4 units in dry bh. =10.92 min ( total paint dry time in the dry booth per unit ),

     while, the 260 units one: 600 min. ÷ 260=2.31 min x 4=9.24 min per units being dried into the dry booth

     Ironically, at other 'B' group factories, two factories whose productivity as per the same working time ( 600 min a day) is at

               20 units different or none; in these case: 600 min ÷ 210 units ( high output a day )=2.86 min(tack time a unit ) x

               4 units ( into dry oven )=11.44 min a unit ( total drying time ), whereas, 600÷190 units( low output a day in the

               same factory, but different buyer )x4=12.64 min a unit.



Why: The paint is just chemical material, thus it can't be adjustable for the quality forcibly, but following the paint suppliers'      standard requiring ( drying time into dry oven, which is also very delicate for total output per day ).

      However, in every society on the earth; there's a theory, so-called, 'Cause and Effect relationship(μ)' as always.

      Provided that factory intends to produce more containers, it's inevitable for paint defects being taken place because they have to ignore paint drying time into dry oven whereas paint drying time under water borne paint system is seriously important / delicate directly. 

     On the basis of actual production layout of the factory and the result of paint dry time per unit calculated above;

     There's a geek gap between the paint suppliers' requiring paint dry time into the booth noted in item (b) above and actual its

     keeping time calculated rightly:

     As the result; at 'A' group factories; high productivity a day: 9.24 min a unit for the drying time, or low productivity: 10.92 min

     a unit:

     Hence, when comparing both data vs. the paint suppliers' requiring time; it resulted in insufficient drying time / improper

     standard against the paint suppliers' standards:

     As radical norm of the keeping the paint drying time requiring by the paint suppliers as below:

             CMP: minimum 12 min to 15, KCC: 12 ~15, Dowill: 12~15, even Kansai more longer:15~25 requiring their standard


             A group factories: 10.92 min or 9.24 min or B group one: 11.44 min. and 12.64 min


How: since few serious paint defects such as pin hole, blister, boiling, etc under water borne paint system are related absolutely to

      the paint dry time into the dry booths; as well we can calculate total drying time into the dry booths a unit easily, comparing

      it against the paint suppliers' requiring standards also, presuming such paint defects to be taken place on the analogy of total

      paint drying time calculated by both logical basic data; tack time a unit out of total output a day and nos. of units putting into

      the paint dry booths.

      Specially, the Zinc on the steel substrate is very important; we have to strive to keeping of correct DFT(30 microns) specified
     in the drawing. ( after painting / drying; we can't realize its DFT unless the PELT is utilized to check the DFT.

      supposing that the ZINC DFT is lower than 30 microns, the drying time is to be shorten as well as total output a day might be



Results: On the basis of actual checks, of course, 'B' group factories' paint quality has been much stable substantially in

         accordance nearly with the paint suppliers' requiring dry time into the dry oven.

         consequently, we of the technical opinion that the total optimum dry time of the container keeping in the dry booth is

         quite essential.


d) what kinds of paint defects to be happened in case of NO standard drying time:

   As drying time into the dry booths is seriously important under WB paint system ( due to slower evaporation  than the solvent borne chemically ),the best way is to harmonize both the total output a day and drying time into  the dry booth pursuant to the paint suppliers' standard requiring time as correctly as they can.

   Whereas, we can detect the following paint defects cases in production factories as well as it can be detected at depots absolutely as below:

   eg., this is the reason why paint pinhole can be reached to the rust on the steel substrate as under-sketched:


As a token of this theory; the flash rust shown on the steel surface on the photos as attached:

  pinholes : paint was not dried completely into dry booth as per paint supplier's standard.


e) In case of flash rust on whole interior panels after having water -spray test done:

- Testing frequency: 1 of 100 units; test time: 2~3 hours after off-lined and the sealant surface dried.

- Before water test, all the inside should be checked and cleaned for steel debris, wooden chips and undercoating contamination.

- Test procedure: spray the entire interior surface by use of tap water and close the door, then seal with tape with signature of inspector including date testing.

-Check the water tested units after 3~5 days, interior TSR, roof/side/door etc. if rust or paint defects are noted.

  l  in case of Flash rust:

-Checking method: visual checking and cutting checking by use of a knife.


 The purpose of this test:  to detect any of possible ingress of water through any pinhole likelihood on the top surface of paint.

    Countermeasure  1) in case of partial flash rust, sand brush, then to repaint by solvent borne paint.

                                      2) in case many flash rust; owner is to be rejected on such unit.


 Note: according to paint suppliers' technical opinion; this test is to recommend technically to perform after 1 week later from unit being lined off against. 2 components curing from chemical reaction of the Epoxi paint.

       In addition, this test is not compulsory and NO international standard rule, but being subject to owner's  decision during contracting with box manufacturer.


l  Penetrable Pin-hole

FYI, a
t the depots in Busan and in Qingdao; did we detect these serious defects during auditing as attached


           This unit was produced in 2016 in China;


many blisters on the surface ( zinc was not dried completely before top coating ( interior )


The following unit was produced in 2017 in China.


   Besides; the examples of the painting defects under waterborne paint system:

  Paint surface cracks : due to high DFT, countermeasure: sand papering, then re-painting

  Serious intensive ext. top coat pin-hole (sponge pin-hole) : due to insufficient drying time)

Countermeasure: optimum DFT and keeping standard drying time. at partial area: sand papering /



Water blister due to improper mixing water ratio.


Top coat surface crack: due to high DFT ( 180~230 microns )


    Mid-coating pin holes due to high DFT ( 140~160 microns and insufficient drying time by high productivity

Air bubbles due to high DFT and zinc was not dried completely before int. top coating.


    Zinc sponge phenomenon due to NO zinc drying ( not dried before mid coating) under high speedy production

 This report has been made by the under-named without any of prejudice on the base of actual data taken on the spot.

-------------------------------------------------------End report-------------------------------------------------------------

Reported by H.J. Henry Kim, CEI International