All of our 31 plants around the world are operating and we are continuing to meet our customers' needs.
Overall, our sales were sequentially up 5% with all regions or segments showing revenue growth, which was primarily being driven by higher volumes as our businesses continue to come back from the negative impact that COVID-19 had on our end markets.
The sequential increase was strongest for us in EMEA as we saw an increase in sales of 13% compared to the third quarter.
Overall, our sales were down only 1% and our volumes were relatively flat but there was a difference when looking at this by region or segment.
The only area that showed an increase in volume sold was Asia Pacific, which had an increase of 8% versus prior year, primarily being driven by higher sales in China and India.
Americas was down 2% on volumes, primarily due to our end market still being impacted by COVID.
And our global specialty business volumes were down 9%, primarily driven by lower aerospace maskant sales.
I also want to point out that our ability to gain new pieces of business and take market share contributed significantly to our performance as our analysis shows that we had total organic sales growth due to net share gains of approximately 4% in the fourth quarter of '20 versus the fourth quarter of '19.
So while we were overall relatively flat in our volumes, the 4% net share gains made a big difference, as did our Asia Pacific growth, helping to offset the negative COVID impact on our end markets.
As we said previously, we estimate we'll take approximately two more years for our end markets to fully return and some markets like aerospace, which makes up about 3% of our sales, may take up more time than that.
The first was a private company called Coral Chemical based in the US that we purchased in mid-December for $53 million net of cash acquired.
Coral had approximately $37 million in net sales and approximately $5.5 million of adjusted EBITDA in 2020.
For this acquisition, we also expect to achieve annualized synergies of approximately $3 million over the next two years.
Also in February, we bought assets related to tin-plating solutions, primarily for the steel end markets for $25 million, which will add full-year net sales of approximately $8 million and approximately $4 million of full-year adjusted EBITDA going forward.
So we are pleased with the strategic additions to our product portfolio, which we estimate will add about $11 million of EBITDA in 2021, which equates to an approximate 7 times EBITDA multiple purchase price for the combination of these two acquisitions.
Synergy achievement also was a factor on our results as we achieved $18 million in this quarter compared to $5 million in the fourth quarter of last year.
In addition, our strong operating cash flow of $66 million in the quarter allowed us to reduce our net debt by another $24 million for the full year.
And for the full year, we reduced debt by about 12%, in addition to making the Coral acquisition.
And reflecting upon the full year, the past 12 months have been challenging due to COVID-19, but I'm very pleased with our overall performance.
And we were able to realize $58 million of cost synergies, which exceeded our previous estimate of $35 million.
We also made these two additional bolt-on acquisitions, which will add approximately $11 million to our adjusted EBITDA in 2021.
And even with those acquisitions, we were able to reduce our debt by 12% or $94 million.
Despite these short-term headwinds, we feel positive about 2021and continue to expect a step change in our profitability with over a 20% increase in our adjusted EBITDA from 2020 as we complete our integration cost synergies, continue to take further share in the marketplace, benefit from a projected gradual rebound of demand in our end markets and see the positive impact of our recent acquisitions.
And I look forward to continuing to be involved in the company's bright future as Chairman of the Board following my retirement.
Q4 net sales were up 5% sequentially as all segments benefited from a gradual increase in volumes and net sales were down only 1% compared to Q4 2019.
For the full year, reported net sales increased 25% in 2020 due to the inclusion of Houghton and Norman Hay.
But on a pro forma basis, sales were down 9%, primarily on lower volumes due to the global downturn in economic production as a result of the COVID-19 pandemic.
Gross margin of 36.8% in Q4 is up from 34.8% in Q4 of 2019, which was deflated somewhat due to inventory adjustments for purchase accounting for Norman Hay.
Excluding these adjustments, we estimate Q4 of 2019's gross margin would have been about 35.3%.
Our sequential gross margin was down somewhat from Q3, reflecting a one-time benefit to gross margin in Q3 of approximately 0.5% and current quarter pressure from rising raw material costs and product mix.
On a full-year basis, gross margin was 36.2% versus 2019's 34.6%.
Excluding similar cost adjustments of Q4, we estimate our gross margins in 2020 and 2019 would have been 36.3% and 35.7% respectively.
We expect to head toward the 38% gross margin area later this year as we further realize our combination synergies and manage prices to offset rising raw material costs.
On both the GAAP and non-GAAP basis, our Q4 operating income improved significantly and our non-GAAP operating margin of 11.3% is up 1.7% versus Q4 last year, reflecting a sequential recovery in sales this year and our combination synergies and the cost-saving actions we took to mitigate the impacts of COVID-19.
Our reported effective tax rate was an expense of 4.9% in Q4 of '20 versus a benefit of 18.2% in Q4 of '19.
Excluding various one-time items, our Q4 effective tax rates would have been approximately 30% and 24% respectively.
For full-year 2020 and 2019, we estimate that our effective tax rates, excluding non-core and one-time items, would have been approximately 25% and 22% respectively, in line with our guidance for this year.
For 2021, we expect our full-year effective tax rate will be in the range of 24% to 26%.
Our non-GAAP earnings per share of $1.63 for Q4 is up 22% from $1.34 in Q4 of '19, due primarily to the improved operating income I discussed earlier.
Our full year non-GAAP earnings per share of $4.78 was down from $5.83 last year but ahead of consensus of $4.67.
Our Q4 adjusted EBITDA of $65 million is up $4 million from Q4 last year and up $1 million sequentially.
And our full-year adjusted EBITDA of $222 million is ahead of consensus.
Our Q4 adjusted EBITDA margin of 17% is up 1.5% versus 15.5% last year and on a full year basis, our adjusted EBITDA margin increased to 15.7% from our 2019 pro forma margin of 15%.
Operating cash flow for the full year was a record $178.4 million, allowing us to reduce net debt by 12% to $717.3 million, paid $53 million for the Coral acquisition, net of cash acquired and pay approximately $7 million in dividends.
As a result of our prudent capital allocation, our primary leverage covenant of net debt to trailing 12 months adjusted EBITDA continues to improve and was 3.2 times at year end 2020 versus 3.5 times last year.
We expect to be at our target level of 2.5 times net debt to adjusted EBITDA by the end of 2021.
In addition, our cost of debt continues to benefit from the current interest rate environment with our borrowing cost under 2%.
In 2021, we expect to see a greater than 20% increase in adjusted EBITDA and further expansion of our margins as we realize the full synergy benefits toward the end of the year.

Summary:
And I look forward to continuing to be involved in the company's bright future as Chairman of the Board following my retirement.
Excluding similar cost adjustments of Q4, we estimate our gross margins in 2020 and 2019 would have been 36.3% and 35.7% respectively.
Our non-GAAP earnings per share of $1.63 for Q4 is up 22% from $1.34 in Q4 of '19, due primarily to the improved operating income I discussed earlier.